Cheating is abuse. When someone tells you they're cheating, and their partner doesn't know, they are bringing you into their abuse behavior. If you don't say anything, you'll become complicit if you know both people.
If it is someone you really don't know, and they and their spouse don't have a relationship with you, and you are not part of their social circle, then I would say that you really don't have as much a responsibility there.
Swap this around however, let us say that it was your brother-in-law that was cheating on your sister. Would you tell her?
A lot of posters have given a lot of good advice here. Nobody in my social circle told me anything. I understand why, but I don't have anything to do with any of those people and never will again. Nor does my spouse. Our marital counselor was very good about explaining how people become complicit in bad behaviors. In many cases they would rather gossip about it, if they are not truly your friends, with other people, than to actually do want a true friend would do.
If you are truly a friend to this man, do what a true friend would do. Don't delay, just do it.
He is already wading through shit up to his neck, he just doesn't know that it is shit that he is wading through, let him know what he's wading through.
Your sister may not like it, she may never speak to you again, and that may suck. I have a brother I have spoken to very little over the last 20 some years because of his cheating on his wife, but it is about more than the cheating. It is because of the way he treated his wife after he was discovered, during the divorce, and after the divorce. He was a verbally and psychologically abusive asshole during the entire process, she never deserved any of that. Being a betrayed spouse myself, I really don't have any room in my life for people who use anything to justify their cheating.
