We are in a profession that helps a variety of clients by explaining, coaxing, having the patience of Job, fighting the good fight, being Good Samaritans to guide taxpayers along the path.  But I would wash my hands/disengage from a client in primarily three situations: refusal to pay the prep fee, deliberating taking an unethical position after being explained the laws, and when guiding a client across treacherous waters to safe harbor, client pushes you out of the boat.  My interpretation of the OP was the client may have known the IRS letter was from his own doing, blaming the OP (for some angle like P&I coverage maybe?) I'd be curious what happened after the OP explained 2019 was not his preparation, did the client offer a mea culpa?