I would not consider a mere signatory to be a "customer" for any purpose, partially because the CIP regulation would not and partially because I would not want tellers believing the person was a "customer" if they presented a check for cash.
However, the difference of opinion only consolidates the advice you were offered: Your bank needs to make and document the decision.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.