Shigeo Shingo, who popularized the methods, made the following distinction:
Mistake-proofing - done to the PROCESS to prevent mistakes from occurring, stop the error from further processing, warn that the error has occurred, etc.
Error-proofing - done to the DESIGN to prevent assembly errors. Examples include adding design features to prevent upside-down, backwards or reversed assembly; using snap-together features to eliminate fasteners (eliminating missing, incorrect, high/low torque).
However, most people using the terms do not know this distinction and do use them interchangeably. Your auditor may be looking for more of these design controls. The best course of action is ask the auditor what criteria they are using to assess this.