Trust me, I'm an accountant and one of my many functions here is to do the payroll for my company and I have for years. They can reverse it as easily as they deposited it. It can be at any time and they do not own her any explanation. As was said, if she tries to hide it, she's stealing.

It could be caught today, tomorrow, next week, next time they are audited or any time in between.

One is starts showing up on their quarterly payroll tax forms or they go to mail her a W-2, someone will notice if they haven't before.