She will have a second supervisor, you have to, but how involved they are can vary.
Basically she doesn't have to disclose unless she wants mitigating treatment, i.e. extra time in exams (obviously not relevant here) or someting like that.
But it can be in her interests to do so, although I totally get why she wouldn't.
It can be a minefield. I got in a tricky one where an MSc student disclosed to his supervisor that he was HIV+, as he was going to work part time in a lab at another institution (having told our disability (yeah yeah, but it's all we've got, if I know it seems odd) people). The supervisor, being a green academic himself came to me, I hadn;t a clue, so called the disability people.
They basically said it was down to me to decide what he should do, which as support staff I wasn't happy with, but he was happy to disclose, so we went ahead. I've still not forgiven the disability officer for basically shunting it onto me though.
She doesn't need to say if she doesn't want to and it doens't affect her work, health, and anyone elses, is what I'm trying to say.
And breakages happen everywhere, and the supervisor is an arse. And I would find working with him pretty tricky after this. Is there no one else in the institution who works in the same field? They won't want to lose her and her funding. Good luck.