The answer is simple. I will help sort out the mess on the basis of full disclsoure.

Then I will not act after that. My confidence in the client has gone. The client relationship is one of trust - in the case you describe that relationship of trust has failed.

And it doesn’t mattre whether it happened before I acted or when I acted - I don’t want the company of people who behave in that way.

Richard

Knowing non-declaration of untaxed income = tax evasion = (at best) an offence of dishonesty.

I am interested to know if you will act (or continue to act in future) for any client of yours who has failed knowlingly rather than negligently to make full disclosure. And if your answer depends on whether he was a client of yours at the time of the non-disclosure?

A client in signing the declaration in a tax return is stating that as far as he knows it contains all taxable income. I would personally have no confidence in, and no wish to act for, anyone who had knowingly signed false declarations.

Fortunately as I do not do personal tax return work (only advice for companies/businesses/business owners) I don’t face this problem.