The CIP regulation does not consider a mere signatory to be a customer for the purposes of the regulation. Examples would be an authorized signer on a corporation's account or a clearly labeled convenience signer on an individual's account.
However, most banks adopted a CIP that requires them to ID mere signatories. Review your board adopted CIP. If it requires you to run mere signatories through your CIP process then that's "the law" within your bank; any outside evaluator will cite you for violating your own policy just as readily as they will cite you for violating the regulation.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.