Giving up the girl is not an option. So here's the plan. Initiate a threesome with him and the girl. While he's focused on her come up behind him and start doing him in the backdoor. It will be a traumatizing experience for him and he will be scarred for life. From now on every time he sees your girl or even thinks about her he will be reminded of the time his friend did him in the ass. Like the naked man it works 2 out 3 times. Sometimes the friend likes it or the gf catches u and it gets really weird really fast