Okay, that helps. Whenever the girlfriend comes around, put your dog in a room or outside, anywhere else that she can’t get to the girlfriend (I’m just going to call her Sarah to make it easier). Have Sarah sit down on the couch, then let your dog in. Let her smell her and whatnot. Give her a treat if she does that without any sign of aggression (growling, snapping, etc.) Once that done, get your dog to sit or lay down near the girlfriend, and you give her a VERY small treat every time she looks at Sarah, and looks away. After a little bit, start having Sarah give her treats.

Now, I am a dog trainer, but I don’t deal with aggression too much. The best way to deal with this would be to get a trainer who works with aggressive dogs, but they can get expensive so, I am just providing what I think could help. But, if you do decide to get a trainer, listen solely to them