Okay, I'm going to try to ask this without confusing any of you!

If you have a relative who has stupidly become involved with a married man and you try to talk to her and tell her how wrong, disappointed, etc., and she knows she's wrong but says their in love, yadda. And, you also know the wife of the creep she's having an affair with and she's a nice person:

Do you tell the wife? Is it your place to interject yourself into this painful situation. Does the wife really want to know? Would you come out being the bad guy for telling? Would it change anything other than what is eventually going to happen...discovery, divorce, bitter feelings. Is it any of your business to tell?

Now, my ex cheated on me and I had this "friend" who called me and made me appear at her house while she kept me waiting so she could tell me she had seen my husband driving down the road with a woman snuggled up next to him. I didn't appreciate it, it hurt me and I felt like she almost wanted to make herself powerful by knowing and then, revealing.

When is it right to tell and when is it right to be quiet?