I'm a stepmom and am very active in a couple stepmom forums.
I agree that your girlfriend probably shouldn't parent your son, at least not early on. What I've heard recommended is referred to as the Lady of the House Rule. Basically, it means that if it effects her, she, as a Lady of the House, may step in, but if it doesn't, she should stay out of it.
He's not doing his homework? Not her problem. He's got the television blaring and she's trying to sleep? She can tell him to turn it down if you or your wife aren't around. He leaves dirty dishes? Depends. Does she need that dish? Is it a habit? Does he leave things around until they attract bugs?
Does that make sense? Good luck!
Me: 30yo wife
Q: 29yo husband of 5 years
(The Divine) Miss M: 27yo mutual friend, GF of Q just recently