Presuming that the amount is significant, (You pick the tax amount that becomes significant) I would have a conversation about amending 2016 - 2020 returns to correct the error and observe their response.

If it is a big enough error that I think the only reasonable way is to amend, and they refuse, - Probably are no longer a new client of mine".

If the tax effect is insignificant, you could do it your way if you wish, even though we know it's not "correct"