“If the restaurant isn’t paying the waitstaff, it ain;t the customers fault”
You know that the IRS assumes a certain tip when calculating this person’s taxes, right? So, she is going to LOSE money for this (apparently good) work. Are you OK with that?
The IRS assumes squat, that just make a bigger tax return anyway.
The restaurant in the employer.
Pay what you owe and leave. It’s the best way to get the waitstaff a raise.