Except of course that isn’t the quote. The ACTUAL quote is: Assume that the customer is right until it is plain beyond all question he is not.
When the customer is demanding you make an ice coffee for the 4th time because you can’t manage to the coffee defy basic rules of specific gravity it is plain beyond all question that he is not.
I see your point. However, at that I would leave it to the manager...Unless she was but still, I’d tell him to come in and make it the way he wanted. At that point I’m sure he would have simply drove off.