I don't know what to do about this, please help.

I have never made an actual post like this on reddit, this is just a bit urgent so it feels needed.

Basically, I'm 16yo, and my gf also just turned 16. She has a dog named Waffles, that she genuinely loves more than anyone in the world, me and her dog are like one of the only things keeping her happy. But she has a terrible, non-biological dad at her house, that is seriously just an idiot. He treats her EXTREMELY unfairly, and it has nothing to do with me being a kid and not liking parental rules. No, he's genuinely just not a good person. He constantly says mean things towards her and her biological dad (they don't know much about him), treats her extremely wrong compared to how he treats his biological daughter, and she's gotten extremely tired of it. So my gf is deciding to move in with her mother. In the past, her non-bio dad has always said she can keep the dog whenever she moves out, or buys a home or wtv. But all of the sudden today, he says "I think I could find a better home than here since your leaving, because I don't like her." Yup, he's now trying to get rid of the dog. The same dog that's keeping her happy in that house with that man. I'm on here because I genuinely need real advice to give her on persuading him to let her keep it. It's just getting to the point to where I am starting to genuinely despise him for even breathing at this point. I don't know what to tell her.. Please help if you care.