I have a client that suggested he could make his cash in the bank disappear. Is it okay or would the IRS suspect something?

I would not represent that client. The fact that they are prepared to commit fraud is a serious red flag, and collection cases get referred to IRS Criminal Investigations. If you do decide to keep the client, I would require them to explain what would happen to the money. If they could spend it on legitimate allowable expenses, then its fine to spend the money legitimately. However, if they plan to hide the money someplace or in someone else\’s name, it is probably a felony. I would explain that under IRC 7201 this would be considered tax evasion (evasion of payment) and could result in prosecution.
Either way, if you know that the taxpayer has assets that they are not disclosing, you are complicit in their crime and could face the loss of your license. It is better to not be involved with these types of clients.