I know this is going to sound easier said then done but it is best for you to cut ALL ties with this guy because if you don't more than likely you will 1. Resent him for what has happened, 2. Resent his girlfriend and him, 3. Feel hurt when you guys hang out and even worse develop stronger feelings for him. Like I said I know this is easier said then done but in the longer it is best to cut ties now and deal with the hurt and move on then to hang out with this guys and ultimately feel let on which will hurt you more and more. Just my opinion anyway. Good Luck!!

Jenn