Well it souds like this guy has issues. I mean if he has a girlfriend then hoe do you know he likes another girl. And have you ever seen her. He could have a fake girlfriend to make other girls jealous. Guys have to feel superior.
It depends on how much he likes his girlfriend. The guy can not like his girlfriend at all, thus he will like another girl.
You let him go or you keep him and tell him that "I want you to like me and not anyone else. Like boyfriend and girlfriend like".
If you want him enough to go into that circle, ask him how serious his relationship is.
I think you should make sure if he likes you and make sure he doesn't have a girlfriend. If he doesn't, ask him out or make it clear that you like him so he asks you out.
If he has a girlfriend, and he likes you, then surely he is not the kind of guy you want to be seeing? I think it would get messy and you would loose friends if either you, or the guy in question tried to change anything.
You Make Him Choose
If you like a guy and he has a girlfriend then you should back off and only try and be his friend.
are you sure you want a guy that likes someone other then his girlfriend. lets say the guy and your friend break up and you and the guy get together then another one of your friend comes along and he likes her instead of you. how would you feel? i think you should tell your friend about it.