While visiting a client to deliver their 2019 tax documents, one of the owners approaches you and asks "my Brother In Law told me I didn't need to claim money as taxable if it was cash, so in my first year of business, which was 2015, I didn't claim $50,000 in cash I received. Is he right?"

You tell the client "I will get back to you on that. I am going to go back to my office and research the item. "

When you get back to your office, you realize the client's gross income for 2015 was $190,000.

What will you tell your client? What laws govern this situation? What penalties are possible?