Why does she have to return it? The employer can simply fire her, if she won’t return it. She very well could have spent it.
A decade or so back, had a coworker who was a mid wage worker, received the Year end bonus that was actually for the VP of the company. He didn’t question it and immediately put in for vacation and went on a fancy cruise with his wife.
When they discovered the error he was not fired, but had spent all the bonus money. He had to do payroll deduction until it was paid back. HR department was a bunch of clueless nincompoops so it was not surprising.
Fire her because they fooked up? Good luck floating that one in front of a judge!