I wouldn't normally say this because so many people do it for too many trivial reasons ....but you know if I were in your shoes I think I would want to consult with a lawyer.
Is there a written contract between the two of you? If not I would really want to consult with a lawyer and maybe even with law enforcement/county animal control in regards to the cruelty this woman heaped upon the poor sulcata.
Basically it sounds like she's an unfit mother & should never get this tortoise back for any reason!
She probably shouldn't have the others that she's still got either.

I hope one way or another you do get to keep this tortoise!
Good luck with that & keep us updated please.
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PHRatz