Well, that's quite a situation. He does seem a bit flakey, but maybe he had his reasons for just up and leaving like that, though it doesn't come off well. Maybe he felt that was the best way to go? It seems really immature though to not even leave a note or something, sounds to me like he didn't have the guts to say goodbye. And maybe he'll return, but it's unlikely. I'd advise you all move on as soon as you can.
I have no experience whatsoever though. Your mileage may vary.