Let me get this straight. The dad is moving in with this...person (I won't say woman) and her little bratz, and his daughter is disabled, and clearly is not keen on the idea, and he is basically forsaking her for a bullying slut.
Yeah, the lady is a bitch, but the dad sounds like a selfish horndog. Either that or very very naive.
Your friend needs to think of herself right now. If she needs to get away from the situation, she should, and she should just ignore the guilt trip they are undoubtably going to send her on. She needs friends to take care of her, and you sound like a great friend, so maybe the situation isn't as hopeless as previously thought.