I once saw a discussion on your site, and cannot remember where it was. The question was that a guest was machmir in a certain issue of kashrus, I think it was regarding whether something was cheilev, and the host was not. The guest said he was not worried that the host would give him the thing that the guest was machmir on, because he knew the host was a tzadik. The question that was discussed was that this is mashma that it was only because the host was a tzadik that he wouldn't give the guest the thing that the guest was machmir on, which is mashma that this is only a middas chasidus: so if I am not machmir in chodosh, for example, and I am not noheig middas chasidus, I can serve him chodosh.
This issue was raised in our Insights to the Daf on Maseches Beitzah 23b.
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Beitzah 23b
Another interesting question raised by the Acharonim is whether one who maintains that smoking is forbidden on Yom Tov may light a cigarette for someone who is lenient. The KESAV SOFER (ibid.), based on the SHA'AR HA'MELECH (Hilchos Ishus 7:12), writes that if one person maintains that a certain practice is prohibited and he helps another person, who maintains that the practice is permitted, to do that act, he transgresses the prohibition of "Lifnei Iver." Therefore, one who maintains that smoking is prohibited may not help another person smoke on Yom Tov, and he must treat cigarettes and other smoking implements as Muktzah. However, the Kesav Sofer adds, if he maintains that the Halachah permits smoking but he personally is stringent and does not smoke, then he may light a cigarette for someone else who smokes.
Thank you so much for your response. It adds a great deal to the sugya.
My question was the reverse of your response: I had asked about a meikil giving something to a machmir. Baruch Hash-m, I did find what I was looking for, and this is what I wrote:
Serving something to a guest when he is machmir for some reason and doesn't eat it, but you hold klor that it is muttar. Two issues: is it lifnei iveir, and is it immoral. Like if I have a guest that doesn't drink cholov stam, and that's all I have. Why shouldn't I serve him the milk in an old cholov yisroel container? I hold it's muttar, so what's the problem? Yevamos 14b, Beis Shammai used to tell Beis Hillel about the children that according to BS are ksheirim but are psulim according to BH. The Ohr Zoru'ah in siman 603, and the Mordechai in Yevamos, and others, all brought in Ramo YD 119:7, say that it is the hosts responsibility to inform the guest, and that the guest can rely on the host's being forthright with him. The Ritva in Yevamos says there's a din of lifnei iveir to give him beyodayim, but see there note 121 that he brings that others argue, including the Ran here, because you hold it's not assur. The Meiri there says it is not obligatory to inform the guest, and the guest can only rely on being told were the host is muchzok bechasidus. I think the most important mokor for this question is in Chulin 111 about the Cheilev.
As I wrote above, your reference is a very good expansion of this question, at least as far as the lifnei iveir issue, although it doesn't address the issue of immorality where the meikil gives something to an unwitting machmir.
Yasher Kochachem!
Eliezer Eisenberg/Barzilai