If you knew someone that was having an affair with someone else, would you tell the other person about it?
Of course. Why wouldn't I, if I knew someone that I really care ?
Suppose...
Jack is your friend (or ex-boyfriend) and Jill isn't, but Jack is the one having an affair with another woman/man behind Jill's back. What would you do?
It'll break my heart to see someone is havin' an affair with another woman/man behind spouse/lover's back. And, yes I would tell him/her if, one is havin' an affair. Well, that's IF he/she is my good friend....
Jack is your friend (or ex-boyfriend) and Jill isn't, but Jill is the one having an affair with another man/woman behind Jack's back. What would you do?
It applies the same as I've just mentioned above.
Jack and Jill are strangers (or someone you "know", but aren't friends with), and one of them is having an affair behind the other. What would you do?