let him be a douche. i think everyone has to go through it sometime or another. just let him know that it sucks and start blowing him off. stop inviting him out, all that. then when he asks why you and your buddies are omitting him from group activities, just say he did it by isolating himself and prioritizing her every moment of every day over you guys. he'll get the message at some point, and if he doesn't, find a new friend.
people need to find a balance, and they need to find it for themselves. if they can't, AINT NOBODY GOT TIME FO DAT.