This question should be answered by a labor lawyer. The issue is related to employment and not workers compensation. However, I would be careful about the cash payments. If your friend doesn't report it, the employer may and of course the under the table wages will be difficult to prove and to prove he worked those hours.
This is the problem when an employee agrees to work for an employer who shows a willingness to engage in unlawful conduct from the outset. When an employer offers to work someone under the table, they are already showing their intention and willingness to violate the Labor Code. It should come as no surprise then they continue to do so in other areas of the business.
That said, your friend has a claim for overtime and penalties as long as he can prove he worked the time stated. He should find an employment law attorney to assist with that process. There are a couple of ways to do it, and which to chose will depend on several factors. At least a consult with an employment law attorney would be very prudent.
Good luck to your friend, and to you.
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