I'm not a native speaker and I'm trying to find a very specific term for a person who eats at a restaurant (from the owner's perspective).
In my native language, there's a word for 'client' and another word that exclusively means 'person who eats at a restaurant.' 'Customer' and 'client' came to mind, but they don't seem to fit, as they're broad terms (they can refer to multiple products / services, not just restaurants).
I already know of the term 'diner.' But it may also refer to a location, not just a person.
Is there any other term I could use? I want the term to be very formal and I don't know if 'diner' is formal / professional / clear enough.