I would have a talk with him maybe, but I would rather ask him if he really wants to marry his fiance. Maybe he doesn't and this would help him realize that if he doesn't, then he shouldn't. I've met some unhappy couples, where both of them had seconds thoughts about their decision, but they still married each other, because they felt they HAD to, since they promised. The biggest promise is the one you make when you marry someone, not when you engage, otherwise, you would marry the person immediatelly. I don't mean he's right to date another girl and then marry his fiance.

If someone told me something about my man, I would tell them to mind their own business. I wouldn't want to know it from a "friend".

I would be worried for the woman only if she KNEW it and pretended not to, just to keep her man. There are milions of those. Cheating doesn't stop when you marry, if it was present before.

This is such a difficult topic and I found out with time that is better to stay out of this kind of situations, esp. telling the other person about "what his/her partner is doing behind his/her back. Many times they will accept it and you will be the one to be blamed - because "you envy their happiness"