Why do people tell you not to tell the wife when you know he is cheating?

If I were in her shoes, I would want to know all of the facts. I may or may not stay, depending upon whether I could live with what was going on or I had a boyfriend myself ( just kidding). Anyway, why do people not tell you? It seems dishonest, exactly what the cheating spouse is.
In this case there are no questions regarding whether it is happening or not. It is. Also, how could the person telling get hurt?
I think that deep down people don’t tell because they are hoping that if tey are in that situation, no one will spill the beans on them. I can’t laugh and smile in my friend’s face while she waxes poetically about her husband who is sleeping with her neighbor.

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