What if you know someone is cheating?

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.

Well, if it's a friend whether they're doing the cheating or getting cheated on it's generally a good idea to say something about it. As for your case the best approach was to stay out of it. Unless you know one of the parties involved very well then there's no good in saying anything.