It's much the same when people join these weird religious cults. They need to be deprogrammed when they leave. One day she'll realise how much she's hurt you and her grandfather but it might not be anytime soon

One day she might wake up and realise that she's been taken in by an ideology and her friends aren't the wonderful people she thinks they are. She'll have to find out for herself that these people might not have her best interests at heart. They only want her to be a member of the cult to back up their own beliefs.
They advise parents of kids who've joined a religious cult to do everything you can to maintain a positive connection and if you’re able, send birthday cards, letters, and tokens of your love and affection. I'm not sure if I would do this though.
Perhaps you should take her up on the offer of free therapy! She'll be paying for it

If she was my daughter I certainly wouldn't pay for anything else if she's abusive towards you.