[Tripper]

Do you know the guy? He probably has a girlfriend himself...or maybe he's just a total loserface and she wouldn't go near him anyway...

I used to get jealous when I started going out with my girlfriend, and then I realised I could trust her. You should be able to tell after a few months if a girl is gonna cheat on you or not.
I personally think I can see it coming from miles away - My mates have dated girls that to me seemed so obviously a bad choice and would cheat on them as soon as they got the chance and my buddies seemed to be totally oblivious to it...

I wouldn't worry about it. If she has been going on about it being uncomfortable to you I'll bet its nothing to worry about. If she hasn't really said shit like that and always talks about the guy - be a little weary, but definitely don't show it.