being that he already knows that she is taken and he doesn't stop his flirting with her, you're going to have to chill on it. your gf doesn't seem to mind it so obviously it doesn't bother her. you will have to however always keep in mind that he is probably telling her that you're probably a good bf, but he can treat her better. i don't really see anything that you can do other than to keep your gf close and always treat her good so that she will never doubt you. feelings can easily be changed during times of hardship so you must ensure what you have and stick to it. honestly i'm not sure how you would go about talking to this guy because he is probably a friend of your gf too so it's hard. other than that, tell your gf that it bothers you because you're afraid to lose her. if you're open with her, she'll find you a lot more trusting and confide in you a lot more. raverboy
...this is just my perspective on the situation...