I went out with a guy like that. For 7 months, it wasn't "official" but I met his mom, his daughter (several times), his friends, we spent weekends together, new year's eve together (out of state, i might add) & I saw him most nights of the week. Still though, he couldn't "commit" & wanted to see other people. Finally I told him, as much as I liked him, it hurt me too much to be with him without a commitment & I let him go. It sucked. I really liked him, and he really liked me & we could have stayed together but I just couldn't with him wanting to see other people. Just not for me.
I was with a guy friend a few weeks after that and I ran into him at a bar. He was shocked I was with someone. And I got a few emails right after that from him asking who he was, and was I trying to make him jealous, etc. Guess he didn't like getting what he'd been giving me. Oh well.
The point, I think
is that if he's not willing to risk his heart in that (or any) relationship, he might not realize what he has until she's gone. And by then, it's usually too late. It was for me anyway. Best of luck to your friend.