To say she is the girlfriend of a male, is to suggest there is a romantic connection. To say she is a woman friend may or may not suggest that; it may just be his ex-wife's sarcastic way of saying the "other woman" is a old cow.
We don't say "man friend" - a male of any age who is romantically referred to by his female friend is a "boyfriend." Even if he's 70.
There is a big difference between boy friend and boyfriend, by the way. Same with girl friend(s) and girlfriend(s). If they are two words, there is no romance. Bring those words together as one, however, and -- well, you get the picture!