Have you showed her the pinpricks of full grown ball python bites? Most people are shocked to see it's nothing. Explain to your girlfriend the new snake is just scared and doing what it thinks will keep it alive. She's going to calm down. I wouldn't try to return a snake for being defensive. The breeder will probably think you didn't do your research before purchasing the animal, and since their's nothing wrong with the animal, there's no real reason to return it.