Hello Tracey,
It sounds like the only solution is once she regains access to her account, she would then have to buy a new Kindle and then she would have access to all of the previously purchased books.
With any Amazon account you should really enable the 2fa security feature.
If she regains her account I have read online that if you keep Contactacting Amazon, and keep telling them the Amazon credit was fraud based and they will remove it eventually.
Regarding bypassing low level support, keep requesting the Fraud department.