I've recently been thinking about a situation that I was in a few years ago. I found out by accident that a friend of mine's mother was having an affair. (I saw her out on dates with the other man) After mulling over it, I decided not to say anything about it. And soon enough, his parents ended up getting a divorce. To this day my friend does not know that I knew his Mom was cheating.

My question is this, if you know someone has cheated/is cheating when do you tell them? When do you not? If its a friend, if its a relative of a friend?...etc.