Ethics in accounting

Can u explain how it’s unethical by also using what;s in bold.
Scenario 7
You have a new client, Buff One Gym Inc., that is very interested in having your firm provide all accounting and tax related services. The owner has asked if you would perform certain services for a special cash price. He says that he has significant cash inflows that he would like to use to pay off bills. He goes on to say that you don’t have to give him a receipt for the cash work, as he doesn’t want to claim it on the company records as an expense. When you express concern about cash deals, he changes the topic. What action should you take?
Issues: code of conduct, illegal activity, tax evasion, moral conscience, responsible use of power)*