I agree alot with what monkeysugar has said and little_tippler
Perhaps try to change her views on you? Yes he might be chasing your 'turf' but you don't want your bf or ex seeing you as the bitchy gf. Show him that you can be kind to what he calls a 'friend'. It might be her with the idea and he might just see her as a friend.
Also as stated, the way he doesn't want to commit is pretty crappy, yes it does sound like he just doesn't want the responsibility of the relationship so he can just go off when/if he finds something better.
As cythetiq said Move on up, if he wants to follow you he will