You are not legally required to work for anybody. So no, you are not legally required to prepare his amended returns.

Ethics is a personal viewpoint that varies from person to person. So you would need to ask YOURSELF how your personal ethics should affect the situation.

Personally, if it was my mistake, my ethics would move me to correct my mistake - either by (a) preparing the amended returns myself, or (b) pay for the cost for somebody else to amend them. However, before you do either one it sounds like you need to consult with your lawyers - probably the one that advised you not to speak with the client and your Error&Omissions insurance.