I agree with perna on most points.

Of course it concerns you and you worry about it, that's natural and what loving gf or so wouldn't worry about their SO getting taken advantage of and it seems like, used? Going by what Perna said though, even though you're going to be stressed about it, you can't change him or make him do anything, only advise and tell him how it's affecting you. I mean if he's enabling his friend now and won't change it, how will it affect you going forward? I guarantee it won't get better if he doesn't change that and step up.

As for the 26 yr old, he's a grown man alright. But yes he's immature and irresponsible.