She can complain to the state DOL. However, I hope she's reporting these cash payments to the IRS and paying her taxes on them, otherwise she's going to be in just as much trouble as the boss.
It is not illegal per se to pay in cash; it's only illegal if the appropriate taxes are not paid on them. She has as much responsibility to pay her share as the employer has to pay his. If she's doing that, she's fine no matter what the employer does and she should go ahead and report him to the state. If she's not, then I'd suggest she get right tax wise before she does anything about the employer.