Tell her how you feel. This cant go on forever.... worse to worse ask her to choose between one of you....... What will happened if she one of those day tell you...that guy got no GF but he need it so she gave it to him!!!!
Your choice.
u need to tok to her and confront the guy..... at times try and use reverse pysco... sometimes it works.. pretend not to be bothered with her and then see her reaction.....
otherwise, go threaten the guy and make sure he get the Fcuk off
come on la. It is only normal for you to be jealous what.
However, u need to be more involved when your gf meet up with that guy.
by doing so, it shows your commitment towards both your gf and that your concern about your gf's fren (that guy).
Maybe, during the sessions you will notice how caring and good your gf is towards her frens (guy and gals) and also make your presence felt by that guy.
Believe that guy will not be so *thick skin as to request that you cannot be around when he meet up with your gf...