When you saw your friend's hubby/partner with another girl

Firstly I would speak to him while he was with this other woman, because it could be she didn't know he was married. I'd let him explain himself, I'd suss out whether he was being genuine with me, if he wasn't I'd tell his wife. Mind you it could all come back on me and I could lose two friends but if that happened then they weren't real friends to me after all, so no great loss and I'd let them get on with ruining each others lives!!

I certainly would tell my friend what I saw. What kind of friend would I be if I kept a secret like that? Also, to make sure she'd believe me, I'd take a picture of the two of them with my digital camera, and show it to her. If I was really annoyed with him I'd likely publish the pic on the internet or something, so there wouldn't be any doubt in anyone's mind what happened.

I dont do that until i trully find out the relationship....

No, I would not do that. I will not interfere my friend relationship unless she ask for it. And I don't really know the reason why my friend's husband with another girl. She can be his working partner, or just friends, or maybe even a relatives. Either way, I wouldn't interfere....what if I'm wrong, it will cost not only my friendship but maybe her relationship too. So, I'll let my friend be the judge and see it for herself...and just support what ever decision she'll make.

Honestly, I would not do that. What I would do is confront the guy. If it is true then I have no right to tell my friend about it. It's their "thing" as husband and wife. I would let him tell my friend about it and give him a deadline. If he doesn't do it, certainly ill tell my friend.

I would not tell my friend. Telling my friend would only cause things to get worse, whether its a misunderstanding or not :p