I am so sorry to hear your plight, that is truely horrible. It goes to show just how unchrisitan that cult is. The bible itself, says that a man that does not provide for his own household is worse than one without faith. To mean that scripture doesn't just mean fiscally, but emotionally also.
Maybe your daughter could put that in her letter, that he is not living up to his God given responsibility as a father. She could quote Ephesians 6:4, which basically says fathers shouldn't provoke their children to wrath. Well, I don't know of anything that would provoke a child to wrath more than being completely abandoned by the one who is supposed to be their support.
This guy sounds like a complete idiot and he is hiding behind his so-called father as an excuse for his lack of parenting. It is sickening.
I hope your daughter finds the solution that will make her happy, you as well. That is a very hard thing for a mom to watch, her child in pain.
Good luck, Mama