We class you as some of our closest friends and understand that having been so ignorant of common protocol you may find our stance comedic but let me debase you of that idea and insist this is normal and indeed expected behaviour when dining at a better class of restaurant.
I'm going to assume he means 'disabuse' rather than 'debase', but either way, it really, really isn't. Charitably, is it possible that someone has played a practical joke on his obviously desperate social climbing tendencies and told him this was an unspoken rule? And that you are supposed to down port like a shot?
Some people are desperately ill-at-ease in restaurants, and probably do think there's some unspoken set of rules that those in the know adhere to. My dad, despite the fact we've taken him to lots of expensive restaurants down the years, still leaps up the second he has eaten the last forkful of pudding (everyone else is still eating, as he's a bolter), rushes to the desk and tries to pay the politely baffled front of house person, apparently under the impression that this is what you're supposed to do. My MIL is terrified of waiters, and will order her food speaking to me or DH in an undertone, so we can pass it on to the again politely baffled waiter, because apparently we speak Special Waiter Language. My mother has a special 'restaurant voice' she uses to waiters which is several social registers above her own.
I bet it wouldn't be hard to convince any of them that this was an unspoken but unbreakable rule.
OP, you may also pass onto your friend that in my glossy London days, I used to eat in a lot of very high-end places. I've had excellent service, including getting a table without a reservation in a fully-booked restaurant, extra dishes and drinks (sometimes something new being worked on by the kitchen, sometimes from the staff meals, which can be fabulous in some restaurants), and, on one occasion, being taken out of a long queue that included a famous TV chef by the maitre d' and taken straight to a table when I didn't have a reservation, all by being nice to front of house and waiters, tipping well (but not insanely), recommending them to other people, and really appreciating the food.
I kind of enjoyed bypassing the annoying TV chef.
I have never brought anyone a doughnut.