I don't see the big deal here. People act differently around their friends, and often differently with different friends. If you just don't like the person she is when she's around this gay friend, then don't hang out with them. My husband doesn't like the way I act around my best friend sometimes, so he doesn't hang out with us very often. It's fine, I get it.
Sounds like you either have to choose to accept her friendship with this guy or find another gf.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi