Basically, it is none of your business and nothing good will come of your relationship if you interfere. It will only drive a wedge between you and your friend.
If he loves this girl, he does not want to know or make a choice between you (as a man friend) and his gf. Maybe you are just jealous of his gf and the time that he is spending with her instead of you. I know you might not like that idea, but it could be true and it could be the real reason that you feel the way you do.
If so, then you need to step back and allow him to have a relationship of his own without you in there somewhere. After all, eventually he will get married and then he will have other priorities in his life. When he finds out, he might ask you why you did not tell him, but unfortunately, damned if you do, and also damned if you dont, in these circumstances I am afraid.