Nobody here has more information than you, sorry man. You're the only one who's personally seen how they interact. But you gotta remember that jealousy can conjure the worst ideas in your head, and that your gf is inevitably going to have male friends. If she's in love with you, or at least loyal to you, she'll shoot him down when and if he asks her out. If she doesn't, then she's going to cheat on you sooner or later anyway. It's inevitable that guys are going to hit on a pretty girl, it's up to her to stay true to you. If she doesn't, you didn't need someone like that in the first place.