Well I guess I'll be the only one here to admit that weed can in fact be a problem, and it would not necessarily be a bad idea for him to quit.

Speculation as to underlying causes of these problems is relevant. They might go alot deeper than just weed, but smoking can only excacerbate them. Of course it can be addictive, of course you can get a weed hangover, of course it can undermine motivation. Just because many of us may or may not experience any or all of these negatives does not mean they not a reality for some smokers. We don't know the whole story, we don't know how much he's smoking, we don't know what else is going on in their lives, don't know the guy. And I for one won't pretend to. It ultimately might be a good idea to quit, but if so, it's going to be HIM deciding it, and no amount of work by his gf is going to convince him (beyond initial efforts or just).