Ethical dilemma:
A women is cheating on her husband. Do we tell him or not?
What is amazing is that this is not a new question but one that has been discussed for centuries. It apparently is a machlokes rishonim between the Rosh and Rambam who argue about whether one must rip the clothing off of someone who unknowingly is wearing a shatnez garment - even though it involves humiliation.
The Rambam [and Shulchan Aruch] say yes - the Rosh says no. They would apparently follow the same approach with regard to our question. The Rambam would say that you have to tell the husband in order to stop him from sinning [unknowingly] with his wife [who is forbidden to him due to her infidelity] while the Rosh would argue against that position.
There are other mitigating factors that would tip the scales in favor of NOT telling. [See Nodeh Bi-yehuda 35 and Yabia Omer 2/2]. I recently saw this shyla in a New York times article and quite surprisingly the author didn't know of the very famous tshuva of the Nodeh Bi-yehuda.
Surprisingly.
I thought everybody knew about it.
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A similar question came up about a couple that was not religious but then decided to do teshuva and send their kids to yeshivos etc. But they told their rabbi that they had been involved with another couple in in wife swapping ה' ירחם [which makes them forbidden to each other]. He told them that they must separate. They said that if that is the case - no teshuva and they are going back to their secular lifestyle. Another rav said to let them stay together as long as they become religious and hopefully over time they will realize that they are living in sin and separate. Otherwise - they will continue living in sin and do all of the other sins as well...
Rav Eliashiv permitted the couple to remain together based on the opinion of the Rashash [Ksubos 9a] that in order for a woman to become forbidden to her husband there must be 2 witnesses to the act of infidelity.
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The Tzitz Eliezer [19/37] was asked about a baalas tshuva married to a Kohen who learned that her relationship with a goy before marriage forbids her to her husband. The Tzitz PERMITTED them to stay together - see the teshuva for his rationale.
עי' בספר ואין למו מכשול ח"י עמ' 341 והלאה למקורות.
Moral: Thank G-d we live lives of purity. Those who don't suffer in both worlds. Those who do reap tremendous fruits in BOTH worlds.
אשרי השומר בריתו!