amended returns

I know of no circular 230 ethical regs that require you to amend a previous client return. from the jest of your post, it doesn't seem you should.

You don't "advise" ex-clients. You might want to "notify" them of an error on returns you prepared, especially if it was your fault and not theirs. The communication ends there. If they ask you to prepare the amended returns, then they are again clients. If they ask you to pay the fees of someone else they hire to correct your mistakes, think twice before declining.

I suggest an *immediate* call to your Errors & Omissions insurance carrier. They'll advise you as to your legal responsibilities.

"I am instructed not to speak to the ex-client by attorneys involved in litigation."

So, apart from the other questions, how would you amend returns without speaking to the client/ex-client/taxpayer?

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.