I think the biggest issue here is that your girlfriend is afraid of the dog, the dog can feel it and feeds off of it. Prior to getting my yorkie, my boyfriend and I watched a bunch of videos about yorkies. This one popped in my mind as I was reading your post:
https://www.youtube.com/watch?v=uwZjILYPhzQ
Maybe your girlfriend can use similar technique. I always had big dogs and I know for a fact its NOT good if your dog snaps at you, even though yorkies are small an't can't do much damage, but still.