I really hate situations like this, because I've been on both ends of them...the jealous boyfriend, and the girlfriends best "buddy". As the boyfriend, you're naturally not going to like your girlfriend being all buddy buddy with another guy (especially someone she used to like), but I alway's say trust her untill she gives you a reason not to. I'm not in your shoes so I don't know exactly how the situation feels, and I'm sure you can explain it as well as you'd like, but going on the info you've provided, I would be a little concerned. I know you said you mentioned it, but you might need to sit down with your g/f to have a serious talk, and let her know exactly how the situation makes you uncomfortable. Depending on her reaction, you also might want to tell your "friend" to back off a bit, and be serious about that too.

The only way to get anything resolved is to get everything out in the open. Don't be a douche or anything and be like.. ya'll can't hang out together.. because that will more than likely ruin your chances with her. If they really are friends, then they have a right to do things together. But, if she really does want to be with you, she should be willing to compromise with you and not see him so much.

Good luck with everything, and I hope you all make the right decisions ;)

IMHO & $0.02