Gemara

Hash-m said "...Chota Yisrael"; Yehoshua asked who had sinned.

Hash-m: I am not a talebearer! Cast lots!

Pesachim 113b: Hash-m hates a lone witness who testifies about immorality. A case occurred in which Tuvya sinned, and Zigud testified about him in front of Rav Papa. Rav Papa lashed Zigud.

Zigud: Tuvya sinned. Why did you lash me?

Rav Papa: "Lo Yakum Ed Echad b'Ish" - you testified alone. Your testimony is Motzi Shem Ra (besmirches his reputation)!

(Rav Nachman bar Yitzchak): It is a Mitzvah to hate Ploni if he alone saw Ploni transgress Ervah.

Question (Rav Acha brei d'Rava): Should one tell Ploni's Rebbi, so he will hate him?

Answer (Rav Ashi): One should do so only if the Rebbi believes you as if you were two witnesses.

Kidushin 66a: Once, they sent Moshe to get Yosef. Yosef came on a different path. Moshe returned and said that Yosef's wife was Mezanah. Shmuel told Yosef 'if you believe Moshe, you must divorce her. If not, you need not.'

(Rava): He meant, if you trust him like two witnesses, you must divorce her.

Kesuvos 85a: Rava judged a case, and ruled that the defendant must swear. Rava's wife said 'I know that she is suspected to swear falsely!'

Rava changed his ruling, and said that the claimant must swear.

Rishonim

Hagahos Maimoniyos (Hilchos Edus 4:1): The Sifri (Shoftim 19:16) teaches that one witness does not help for capital cases, monetary cases, Korbanos, lashes or lineage. He only obligates an oath. Therefore, if he testifies in any case when he does not obligate an oath, he is lashed for "one witness will not stand against a man." He is merely Motzi Shem Ra. He may testify in order to separate from an Isur, like the Shali'ach who told the man 'your wife was Mezanah', and Rava's wife, who told him 'this woman is suspected to swear falsely'. They were not rebuked for saying this.

Sha'arei Teshuvah (3:215): If one sees Ploni transgress a Devar Torah in private, he may not tell the Rabim about it. Perhaps Ploni repented! He may tell a modest Chacham who will not tell others.

Sha'arei Teshuvah (219): It says (Mishlei 24:28) "do not be a vain witness." If David saw Ploni steal or withhold wages, he must testify about this. If there is no other witness, he will force Ploni to pay or swear. However, if Ploni transgressed Ervah or an Aveirah, there is no use to testifying, even if there is another witness. If Ploni fears sin, David should think 'surely, he repented. Perhaps his merits exceed his sins, and he is a Tzadik.

Be'er Mayim Chayim (15, on Chafetz Chayim 4:4): If Ploni repented, and one shames him in front of others, this is 'making someone blush in public', for which one loses his share in the world to come. Perhaps Ploni's merits exceed his sins, and he is a Tzadik! If one heard from two that Ploni sinned, he may not accept that it is true and hate him. He may only suspect perhaps it is true. Surely he may not tell others!

Sha'arei Teshuvah (219): However, if Ploni is a fool who repeats his folly, it is good that they tell the judges to separate him from Isur. However, if David is the only witness, his testimony would be vain, for we do not rely on it. He is Motzi Shem Ra. Even if Ploni revealed David's sins, David may not say 'I will do the same to him.' If Ploni has no fear of Shamayim, e.g. he cast off the yoke of Shamayim and is not careful about a particular Aveirah that everyone knows is forbidden, one may shame him and speak to his detriment. He is not 'your compatriot.' We expose flatterers to avoid Chilul Hash-m. However, if he just happened to sin, but normally he is careful not to, one may not publicize his sin.

Chafetz Chayim (7): If one tried to rebuke him and he did not accept, one may disgrace him until he repents.

Be'er Mayim Chayim (14): If Ploni was warned, it is as if he transgressed something that everyone knows is forbidden. The Rambam (Hilchos Rotze'ach 13:14) and Shulchan Aruch (CM 272:11) say that one may hate Ploni for one transgression, but he is still 'your compatriot' regarding the Isur of Lashon ha'Ra, unless he transgressed many times. This is clear from Pesachim 113b and R. Yonah.

Sha'arei Teshuvah (220): One who testifies alone is lashed mid'Rabanan. However, he may covertly reveal the matter to Ploni's Rebbi or confidant, if he knows that they will believe his words like words of two witnesses.

Sha'arei Teshuvah (221): In matters Bein Adam l'Chaveiro, e.g. theft, withholding wages, damage, causing pain, embarrassment and verbal abuse, even an individual who sees this may tell others, to help the victim and Lekanei ha'Emes (to stand up for the truth).

Poskim

Rema (CM 28:1): One witness may testify only about a monetary matter in which he will obligate an oath, or to separate one from an Isur. If the Isur was already done, he may not testify, for this is merely Motzi Shem Ra.

Darchei Moshe (2): R. Yerucham (2:2:11(4)) says that one who sees an Aveirah is obligated to testify about it, due to "you will eradicate the evil from your midst." He discusses when there are witnesses, or in order to separate from Isur.

Chafetz Chayim (4:4): If Ploni once transgressed something that everyone knows is forbidden, if he is an average person who normally is careful not to sin, one may not tell others. Perhaps he already repented. If so, the teller bears great sin for disgracing him in the eyes of people. One should not reveal it even to the judges, even if there is a second witness, for there is no use. (If there is only one witness, in any case he may not reveal it, for the judges may not believe him). Rather, he rebukes Ploni softly. All the more so one may not publicize the sin of a Chacham.

Be'er Mayim Chayim (16): R. Yonah permits telling judges only when this can help the victim recover his money. In other cases, one should not tell them, but it is not such a great Aveirah, for then the judges will know that Ploni may not testify or swear until they see him repent. If the witnesses know that this will benefit a Yisrael, they must testify with this intent. The judges believe it only if the witnesses testify in Beis Din. Outside of Beis Din, judges are like regular people, and they may only suspect. The Rema teaches that one may not testify, but always two may testify.

Chafetz Chayim (5): If Ploni is a fool who will not heed rebuke, and he is prone to repeat his sin, it is good to tell the city judges, so they will afflict him and stop him. It seems that one may tell Ploni's relatives, if Ploni will accept their words. The teller may intend only for Kin'as Hash-m, but not for hatred or any other reason. The judges must afflict Ploni covertly, and not publicly disgrace him. This is if two saw him sin. If only one saw, he may not testify about him, for we do not rely on one witness. One may tell Ploni's Rebbi or confidant if he will be believed like two witnesses; the Rebbi may hate Ploni and distance himself from him until he repents. The Rebbi may not tell others, for this is no better than if he saw it himself.

Chafetz Chayim (6): It seems that if Ploni normally repeats his folly, this is permitted even if the Rebbi is not so careful and it is possible that others will find out, as long as Ploni heeds his Rebbi's words and will not repeat his sin. The teller may intend only to help Ploni, but not to belittle him.

Sedei Chemed (Lamed 132, DH v'Hinei): Parshas Derachim asked, how can Semag say that one is lashed? Even if the Lav forbids this, there is no action? Vovei ha'Amudim answered that it is included in Motzi Shem Ra, who is lashed even without an action. Ner Mitzvah says that the Rambam holds that one witness is not believed, so it is false testimony. A Gezeras ha'Kasuv teaches that one is punished like he sought to inflict (death, lashes or money) for false testimony, even though there is no action. I say that even though one witness is like a false witness, he is not like an Ed Zomem. The Rambam says (that we punish like he plotted) only when it is known through witnesses that he testified falsely. Also, what is the source that testimony that does not help is like Sheker? If two pairs of witnesses contradict each other, they are not punished even though their testimony did not help! (I do not understand the Hava Amina to punish either pair. Perhaps the latter pair testified Emes, and stopped us from heeding false testimony. Perhaps the first pair testified Emes, and they did not expect the latter pair to contradict them. Even if the latter pair said that they will, perhaps it was a mere bluff. If two witnesses come, and one testifies, and the other retracts and refuses to testify, surely we do not punish the first! - PF). The same applies to one witness. The Ketzos ha'Choshen (38) was unsure whether Ka'asher Zomam applies to one witness. The Minchas Chinuch (37:4) says that he overlooked the Rambam (Hilchos Edus 21:5), who says that it does.

Chavos Ya'ir (72, b'Sof): A case occurred in which a woman about to die admitted that she became an Eshes Ish (i.e. was Mezanah witrh another man), then she returned to health. Her husband needed to divorce her. The same applies if the Bo'el confessed.

Noda b'Yehudah (OC 35 DH Emnam Mah): If a witness saw that Ploni's wife was Mezanah, and Ploni does not believe him, he should not go to Beis Din. Ploni need not believe him. What will Beis Din do?! If he testifies, he is lashed mid'Rabanan. The Rema permits in order to separate from Isur, i.e. when his testimony might be believed. Shmuel did not rebuke the Shali'ach who told him about the Zenus. The Shali'ach had not yet seen her husband. Had the husband already heard and not believed it, it would be mere Motzi Shem Ra to tell Beis Din. The Chavos Ya'ir did not rule that a Bo'el's admission obligates the husband to divorce her. Even if he ruled so, we do not follow a ruling of Acharonim without reason. Really, he refers to his previous words. Initially, if a woman told her husband 'I became Asur to you', he needed to divorce her. Chachamim retracted, lest she say so in order to marry another man. If there are Raglayim l'Davar (grounds to believe her), he must divorce her. A Bo'el's admission is Raglayim l'Davar. If a peddler was secluded with an Eshes Ish, and there are witnesses of Ki'ur (Raglayim l'Davar of Bi'ah), she must leave him, i.e. the peddler (if she married him after her husband died or divorced her). The She'altos says that she must leave her husband, but he agrees that a Bo'el's admission is not Raglayim l'Davar (if she denies it). Therefore, the Bo'el should tell only her husband covertly. Letter of the law, the Bo'el should just say 'I saw that she willingly had Zenus.' However, it is logical that her husband is more likely to believe it if he sees the Bo'el tearfully confess. If it is embarrassing to admit, this will help atone for his sin. He sinned also against her husband (he forbade his wife to him). He cannot undo the damage, but he must ask forgiveness for the pain. Teshuvah does not help if he does not try to appease her husband.

Chafetz Chayim (4:7, b'Hagahah): One may tell her husband only if it is possible that he will believe him like two witnesses.