Unfortunately, this situation isn't something Square can really do anything about.
If a customer has their bank do a "charge back", the bank is virtually guaranteed to side with them. Even if you have overwhelming proof that the good or service was provided in full and to specifications, the bank will not care. Their sole concern is satisfying their customer.
If the client does reverse the charge, all Square can do is provide the bank with the information they have (for instance, date/time of transaction, whether a chip card was used, if the customer provided a signature in person, etc.). But that is all just data. The actual decision is 100% in the bank's hands. Square has zero say in the matter, and no control at all over the transaction. They're legally required to hand over the funds to the bank.
I'm afraid all you can really do is try to satisfy her the best you can. Perhaps a partial refund, for instance. I'm not sure how she could justify a total refund if she also expects to keep any of the product of your work...
Depending on the good or service, the best outcome might be just to take back what you gave her, give her a full refund, and move on.
