She was estranged from her father. After his death, her mother encouraged her to take the hard drives and USBs. Assuming that his wife was the legal owner after his death and it was in fact gifted her then it sounds legit.
I assume data is property.
In this case the mother already had joint possession (through marriage) of her and her husband's property. While we refer to it as "his USBs" because he probably bought them and used them exclusively, when you're married they are legally owned jointly by you and your spouse.
One exception would be if he did the work on behalf of a company and the company paid for the USBs. Then the company could probably sue to have their property returned. Or if he had a contract saying that he had to delete the data after the contract ended, that could probably be enforced as well.