Gemara

(A reciter of Beraisos): The Makif (one who cuts Pe'os ha'Rosh, i.e. sideburns) and the Nikaf (one whose Pe'os are cut) are both lashed.

Objection (Rav Chisda): Why is the Nikaf lashed? He did not do anything! This is like R. Yehudah, who says that one is lashed for a Lav she'Ein Bo Ma'aseh!

Answer #1 (Rava): It is even like Chachamim. It teaches that one who cuts his own Pe'as is lashed twice (for each Pe'ah).

Answer #2 (Rav Ashi): It is even like Chachamim. The case is, he moves his head to help the Makif. This is an action.

Nazir 57b (Rav Huna): One who cuts the Pe'os of a child is liable.

Rav Ada bar Ahavah asked, and heard that Rav Huna's wife cuts her children's Pe'os. He said that she should bury her sons. For the duration of Rav Ada's life, Rav Huna did not have children (from her) that survived.

Rav Huna equates "do not round the corners of your head" and "do not destroy the corners of your beard." Only those commanded about the beard, i.e. men, are commanded about the Pe'os.

Rav Ada holds that the verse equates the Makif and the Nikaf. Whenever the Nikaf is liable, the Makif is liable. The Torah does not punish children. Since a child is exempt, one who is Makif a child is exempt.

Kidushin 35b (Mishnah): The Lavim of Hakafah and shaving the beard do not apply to women.

It says "do not round the corner of your heads and do not destroy the corner of your beards" - only those commanded about the beard are commanded about the Pe'os. Since women may shave, they may cut Pe'os.

Bava Metzia 10b - Question: If a Chatzer acquires like a person's Shali'ach, how does it acquire for a thief? We hold that there cannot be a Shali'ach to sin!

Answer #1 (Ravina): This is only when the Shali'ach is liable for transgressing. A Chatzer is not liable.

Answer #2 (Rav Sama): There cannot be a Shali'ach to sin when the Shali'ach can choose whether or not to comply. A Chatzer has no choice.

These answers differ about a man who told a woman 'cut a boy's Pe'os for me.' Since she can choose to obey or not, Rav Sama exempts the man who told her. Since she is permitted to do this, Ravina obligates the man.

Rishonim

Rif and Rosh (4a and 3:3): The Halachah follows Rav Huna. If a woman cut Pe'os, or her Pe'os were cut, she is exempt, like it says in Kidushin.

Nimukei Yosef (DH v'Halachah): The Halachah follows Rav Huna, who permits a woman to cut a minor's Pe'os l'Chatchilah. Some permit a man to have his Pe'os cut by a Nochri, woman or child, since the Makif is exempt. This is wrong. Just like a man who is Makif a minor is liable, even though the Nikaf is exempt, if a woman or child is Makif a man, the man is liable, even though the Makif is exempt. Some are unsure whether or not Rav Huna forbids a Yisrael to be Makif a Nochri, just like he may not be Makif a child. The Rosh permits; Rav Huna forbids only to be Makif a child, for the child will later be commanded. If so, likewise, a man may be Makif a woman.

Rambam (Hilchos Avodah Zarah 12:1): One whose Pe'os are cut is not lashed unless he helped the one who cut them. One who shaves a minor is lashed.

Ra'avad: Even though one whose Pe'os are cut is not lashed, since he consented, he transgresses a Lav.

Rambam (2): If a woman cut a man's Pe'os, or if she was shaved, she is exempt. It says "do not round the corner of your heads and do not destroy the corner of your beards" - whoever is commanded about shaving the beard is commanded about the Pe'os.

Tosfos (Nazir 57b DH v'Rav): The Stam Gemara (Bava Metzia 10b) holds like Rav Huna. It could have said that a man told a woman to be Makif a man. However, the Nikaf would not agree, for he transgresses. A minor is not punished, and cannot protest.

Tosfos (Shevuos 3a DH v'Al): It seems that one who shaves (the beard) is liable just like one who is shaved, just like regarding cutting the Pe'os. Even though regarding Pe'os we learned this from the plural "Lo Sakifu", and regarding shaving it says "Lo Sashchis" (singular), regarding Kohanim shaving it says "Lo Yegalechu", and we learn about Yisrael from Kohanim. A Tosefta explicitly obligates one who shaves a man. One who is shaved by a Nochri is liable. We learn from Rav Huna, who is Mechayev one who is Makif a child. He does not equate the Nikaf to the Makif; likewise, we do not exempt a Nikaf when the Makif (or Megale'ach) is exempt. Even though Rav Ada argues and permits a man to be Makif a child even mid'Rabanan, the Halachah does not follow him. The Stam Gemara (Bava Metzia 10b) holds like Rav Huna. Alternatively, even Rav Ada obligates a man Nikaf by a Nochri. He exempts only when the Nikaf is not commanded.

Poskim

Shulchan Aruch (YD 181:5): One who shaves a minor is lashed.

Rema: It is permitted to cut the Pe'os of a Nochri or woman. Some are unsure about this. A minor may be Nikaf by a Nochri.

Shulchan Aruch (6): The Mitzvah of Hakafah does not apply to a woman. Some say that even though she may cut her own Pe'os, she may not cut a man's Pe'os, even if he is a minor.

Beis Yosef (DH v'Ishah): The Rif and Rosh permit a woman to cut a man's Pe'os. Surely, he may not allow this. He transgresses even if he does not help. The Rambam forbids her to cut even a minor's Pe'os. Since Rav Huna was punished, this shows that the Halachah does not follow him. The Ra'avad agrees. It is forbidden, but she is not lashed, even for Hakafah of an adult man.

Gra (6): Tosfos (Shevuos 3a) forbids a woman to cut a man's hair.

R. Akiva Eiger: All should forbid a woman to cut a man's hair, due to "V'Lifnei Iver Lo Siten Michshol"! Even if he could have shaved himself, it is forbidden mid'Rabanan. Perhaps here it is permitted, for she saves him from an additional Aveirah: had he shaved himself, he would be liable also for Makif. Rav Huna permitted a woman to cut a child's hair l'Chatchilah. Rav Ada cursed only because he thought that according to Rav Huna, it should be forbidden. He himself permits mid'Oraisa, and perhaps also mid'Rabanan. This law requires investigation.