My immediate response to any auditor trying to do this would be to ask him which clause he is writing me up against.
There is NOTHING in the standard to say what training your internal auditors must have. As lead internal auditor with a training qualification, I train mine in-house and on-the-job, and they get a damn sight more information and support than they’ll get out of most external agencies.
9001:2015 also clearly indicates that it is up to companies to determine their own competency requirements. If I have documented that Internal Auditors may be untrained then there is nothing they can make stick.