This must be so scary for you op, I really feel for you. Such a difficult situation. You should take it very seriously not brush it off like pp suggested.
I think the idea of not badmouthing the other parent must go out the window to some extent when this happens. I'm not saying you should slag him off but I do think you need to be proactive in correcting the narrative that he feeds her.
I'm not really sure how but some things that came to mind were..
Can you talk to her about this kind of coercive control so she understands, in an age appropriate way, what he is doing. I wonder if there any books or films about this subject.
Point out to her that you haven't said anything bad about her dad. Tell her you are worried that daddy doesn't like you and that she will believe everything he says and will stop liking you and that you are sad because he is making her sad.
Tell her that he shouldn't be talking about you to her and empower her to be able to ask him to stop, maybe have a conversation about this with him when she is there and email him this so you have it in writing.
You obviously have to be careful to keep things factual and remember that she might repeat things to him. Also be aware that she will find it difficult knowing who to believe, if possible back what you say up with evidence.
Tell her the courts decided that he couldn't see her not you, keep repeating this, hopefully this will stop her blaming you. Be factual about what the courts found and about the DAPP etc, this wasn't your choice.
Sounds like you are in regular contact with the school but if you're not I would talk to the safeguarding officer at school "for advice" at regular intervals, your concerns will get logged and there will be a clear record of it.
She is in such a difficult situation, caught between two parents that she loves and wants to please (completely his fault). If possible she will benefit from having someone neutral to talk to about what is going on. Can you push for counselling through the school? (Can't remember if she is getting any support)
It's great that she talks openly to you about what her dad says, this shows that she trusts you and feels comfortable with you. You need to tread very carefully so that this open discussion continues but I do think you need to act.