I would proactively document the hell out of the fact (certified mail? ) that the client has been informed of the need to amend and that any additional penalties & interest assessed 18 months from now when the IRS notices the error are the client's responsibility.   

You are responsible for the penalties & interest up until the preparation of the 1040X for filing (since the error was yours).   I might even consider cutting a check for those, and send it the client with the 1040X.  (Depending on my level of paranoia & the amounts involved...  🙂