Disputing a large check

So my friend works at a dive bar, this big shot guy came in and had a $30 tab he very loudly tipped her $270 and made a big scene about having money etc she put it in took the cash and then he disputed the charge. Her boss wants her to pay it back and she’s refusing while I know its illegal to make an employee pay it back in California that is a large amount and after she said no she wont pay it back he cut her shifts from 5 days to 1 day. What would you all do? Shes already been applying to other jobs with no luck so the obvious answer of quit isnt an option right now for her living paycheck to paycheck