This point is a machlokes between Rosh and Rambam, based on Brachos 19b: Said R. Yehuda in the name of Rav: one who find shatnez in his garment removes it [the garment] even in the marketplace [where there are many people who will see him without his shirt]. They ask from several sugyos that k”h pushes off prohibitions, so it should push off wearing shatnez also? They answer “shev v’al taaseh” [doing nothing to violate the prohibition] is different. We see wearing shatnez is considered an act. (Even though he is not putting on shatnez now, it’s already on him, not removing it is considered an act.)
It’s a machlokes what exactly is the case of this gemara. Rambam’s girsa is “one who finds shatnez in the marketplace” and does not have “finds in his garment.” He understands this is where you see someone else wearing shatnez, you are required to remove it off of him. We see this is shev v’al taaseh in regard to the finder, but because the wearer is an act, the finder must remove it from the wearer. The girsa of the Rosh is where you find shatnez in your own garment. Therefore he paskens in Hilchos Kilayim that if you see someone else wearing shatnez and he is shogeg, the wearer is not considered to be doing a prohibited act, and the finder is just passive (shev v’al taaseh).
Rambam learns his din even when the wearer is beshogeg, as Tur YD 303 brings the Rosh as arguing on Rambam, and in Shulchan Aruch they are brought as two opinions , and the mechaber paskens like Rambam and the Rama paskens like Rosh.
Our case is exactly like that case – the husband will violate by doing an act – being with his wife who is forbidden to him, but he is shogeg, not knowing that she was unfaithful to him. And the one who knows that he is doing an aveira (the current son-in-law) is passive in not telling him. To the Rosh he may remain silent due to kavod mishpacha [being k”h]. To Rambam the son-in-law must inform the husband to stop him from violating a prohibition.
You can say even Rosh will agree here the son-in-law must tell because the husband must divorce his wife to save the wife from a prohibition because she knows she was unfaithful yet she remains married to her husband. But you can push this off because the woman doesn’t know that she’s forbidden to her husband and omer mutar is not worse than shogeg. I can even suggest to the Rambam it’s not certain that he must inform the husband because it depends on machlokes Rashi Tosfos in Brachos there. Rashi s.v. shev v’al taseh says tumah was allowed for nazir and kohen when he comes across a mes mitzvah which he is uprooting [his prohibition against becoming tamei] with an action because of k”h, there is not k”h because he was never prohibited against becoming tamei for a mes mitzvah, just like there is no prohibition for him to become tamei when a close relative dies. Tosfos s.v. Shev argues on Rashi and says it does push off the lav, but Nazir is an exception because he can have his vow of nezirus annulled, and a kohen is an exception because becoming tamei is not a law that applies to everyone (shava lakol; it’s a din that only applies to kohanim). So to Rashi a lav hashava lakol and even if it can be annulled, even as an action is not pushed aside because of k”h, and to Tosfos something that can be annulled or it’s not shava lakol is pushed off because of k”h even by doing an act.
I say that this woman who was unfaithful and became forbidden to her husband is comparable to nazir which can be annulled, because the idea of being annulled means the prohibition can be removed meikara (a priori / retroactively). The husband can do this also based on what Tosfos says in Gittin 33a s.v. v’afkinhu: can a man who is married to his niece (I assume this case was used because it is a case where he will not want his unfaithful wife to be killed-Ed.), and she has committed adultery, send her a get and annul the get after he already sent it to her? The chachamim in this case retroactively annulled their marriage. So can he do this to annul the marriage and make them not married at the time of her infidelity (and hence save her from the death penalty)? Ri says yes, and it will be like nazir. Thus Ri's opinion here is we push off the lav [of remaining with his unfaithful wife] even with a action because of k”h. (Then Noda Biyehuda has a brief discussion that this Tosfos is according to a tana that we don’t pasken like, but he concludes it is like nazir.) (About this sugya – see Rakeffet-Rothkoff, Rabbi Aaron, “Annulment of Marriage Within the Context of Cancellation of the Get “, Tradition, 15:1-2, link: here.)
(to be continued)