Well really he doesn't have to break off being friends with his fellow band mates to date this girl. It's pretty moronic to blow off your friends every time a girl (or guy) comes around that you may or may not have a future with. Also, I don't see a reason why he really needed to quit the band. I know quite a few musicians who balanced school, relationships, jobs and being in a professional band (in fact most of our music teachers at our high school were and still are in bands and went through college while being in these bands).
I know a bloke in the same agegroup who's doing a dual major (law+French), plays in a http://spacesbetweendays.bandcamp.com/ (at least one person is committed to it as a career option)
and one or two which are more casual (bit of money on the side, opportunity to play music). Works and has a girlfriend too, I'm pretty sure.
I suggest you avoid a "The band wants you back" angle, and go with "As far as I can see, you're being a complete idiot, why on earth are you doing things this way?". Level of tact up to you.
At the end of the day, what Mamagato said is true: if he continues with this path (i.e. its not an early-life-crisis or 'just a phase'), he's not worth the trouble.
On the other hand, could there be some drug use (including excessive alcohol consumption) or suchlike within the band? That may be a confronting question, but bands have been known to partake in such activities, and its very relevant.
If that is a factor, would be an entirely understandable reason to cut ties with his band if he's trying to clean up his act, so to speak.
EDIT: What ever else you end up doing, I think its worth starting by asking him exactly what prompted this course of action.