If your mom is that valued, SHE should find another place - and take you with her.

So what if they are short-staffed? You don't have to take abuse; it's not your job to stay.

And if the manager is worried that the guy will stir up trouble - they can talk to a lawyer first and get their ducks in a row. THEN fire him.

Leaving is part of the solution - if nobody, including you, actually DOES anything, then they are being a part of the problem.

hugs
Catty1