from a woman who's been cheated on, had cheating friends, etc. ...
1. mind your own business. seriously. just stay out of it. i PROMISE you ... it will all come out in the wash.
2. if you absolutely, positively MUST say something, say it to HER. not him. you have no idea what the precursor to that convo was. it might have been a joke in reference to something that another woman did the last time they went out, etc. you have no idea. additionally, i don't wear a wedding ring, unless i'm at work. i try to remember it when i go out and notice that i get tons of attention (and free drinks) when i forget to put it on. i never, EVER have the intention of cheating, and never do. however, i never get chatted up or a drink bought for me if i wear a wedding ring, and i love communicating with people and talking at bars (and make it clear i'm not looking).
i've actually told a friend that i wasn't going to wear my wedding ring when we went out for drinks that night because i was low on cash! of course, i wore it, and used one of The Husband's gold cards. haha.
but if one of his friends had overheard me saying i was going to NOT wear my ring to talk to guys/get drinks/whatever, i'd be pissed as hell if he went and narced me out to my husband.
if you're truly bothered by the convo, just tell her what you overheard and ... well, then what? what would you be trying to accomplish by confronting either one of them?
These are my thoughts exactly.