This is hilarious.  You're questioning the ethics of a CPA firm that noticed your client qualified for three quarters of the ERC, something that apparently you missed.  But now you are certain that they are wrong about another three quarters.  People who live in glass houses shouldn't call kettles black.  Or something like that. 

The ethical question now is whether you should continue to consider these taxpayers your client.  Have they asked you to do amended returns?  Are you insisting on doing them?  I think "former clients" is a better description.