I have recently found out that a good friend's husband regularly sleeps with prostitutes when abroad for business trips. About 3 or 4 times a year. They have 2 teenage children and are in a "happy marriage" as far as everyone is concerned. He just happened to let slip to my partner this weekend that he does this when away. 
I am not going to tell her, it's none of my business. I can't stop thinking about it though..... what would you do if you know this sort of thing? I am right to keep my mouth shut, to them both aren't I?