Re: AS9100 and Nadcap vs NAS412
So, he is a customer (2nd party) auditor. I am not familiar with NAS412, but he is correct in stating that conformance (and certification) to AS9100 and Nadcap is no guarantee of FOD-free products. On the other hand, you are correct that conformity with standards normally require a review of it's application and exclude non-applicable requirements.
Typically, the best way to resolve this is to dialogue with the auditor and reason about the applicability of the requirements. Failure to reach an agreement should be followed with a contact to the QA and procurement functions of the customer. After all, enforcement of nonsensical requirements onto suppliers would normally lead to contract renegotiation and potential price increases, assuming that your organization had done proper contract review and assessed and agreed with the customer on the degree of conformance necessary with NAS 412.