Sounds like it's because of your relationship with her. If you were friends with her he would probably call her by her first name to you. Not a big deal, people do it all the time.
I don't see anything wrong with him doing that.
In movies and tv shows, when men always call their woman not by their name but by, "the wife" or "my old lady" "my girl" "my woman" they are trying to sound macho. If they say it to another woman that is just a friend, they are interested in the friend and are trying to minimize the wife to her, usually portraying her as some naggy annoying mom character when it is not really the truth, and he is like a teenager trying to escape her oppressive rule, in an effort to get the girl "friend" who he wants sex from, to be sympathetic towards him and in order to justify his cheating in his own mind so he doesn't feel as guilty. really painful to watch!
It would be bad if he calls her girlfriend when talking to her. Sounds a bit like Tarzan. Nah it's ok. It would be bad if he referred to her as just "friend" instead of g-friend.