If that person has a legal duty-of-care or liability for the diver, then it's absurd to think they shouldn't be permitted to assess and form a judgement over the diver's relative competency for a dive.
We're talking about a situation where some idiot can wander into your business with a plastic card, vast sense of entitlement and a deluded sense of their own competency. (see 'Dunning-Kruger effect').
That person may (and this IS common, believe me) wish to undertake dives that are vastly beyond their actual capability. If they hurt themselves, they'll blame (and sue) everyone else involved. If they kill themselves it's business suicide for the dive operator concerned.
But sure.... the customer is paying money, so that makes them automatically right. And scuba diving is such an over-priced rip off, right?!
God forbid any dive operation might put safety first...in priority over appeasing the demands of divers whose minor financial expenditures grant them a ghastly sense of entitlement...
Again, this points to some sort of fantasy that a c-card constitutes a 'license' granting the holder an entitlement to waltz into any dive operation and demand whatever they want.
A c-card is nothing more than a proof of training. It shows you did a course, once upon a time, and did potentially *just enough* to pass that course. Remember that virtually no-one fails scuba courses...
Also, very few....a tiny minority of divers ever practice their skills post-certification... it's prudent to NOT assume that the diver has retained anything like the level of skill ability they demonstrated at the time of graduation.
Dive pros see this day-in, day-out. The ethical ones stand firm on safety. The disreputable ones pander and grovel to the customer's delusions because they won't ever turn down a quick buck.
When divers visit enough unethical dive centers they start to believe that syphcophantic pandering....and coupled with their gross inexperience... it can create laughable delusions of grandeur.
They get 'insulted' when someone ethical finally says "No" and.... shock...horror .. actually asks them to prove their ability.
Instructors are deemed prudent to assess student diver competency during training. What illogical delusion needs to be raised in order to suggest that they're somehow unqualified to assess a diver's competency after qualification!!?
If you, the diver, were willing to accept an instructor's assessment to pass your courses, then you should also accept the same authority-to-assess outside of the training relationship.
Strikes me as an attitude that you'd only accept an assessment if it were positive and empowering.... but would throw a big hissy tantrum should the very same person suggest a negative assessment that limits what you believe is some illusionary entitlement you possess..