Is there one word for people to substitute "people who eat out in restaurants "?- whether in quick service restaurants, eateries, or high end restaurants. Terms used in business are preferable.
Diner: "someone who is eating a meal, especially in a restaurant"
business: clientele: "the customers of a shop, bar, or place of entertainment."
A person who frequents restaurants rather than dining at home.
One who goes to or attends restaurants.
2009 September 6, Seth Schiesel, “All Together Now: Play the Game, Mom”, New York Times: Sure, the result won’t be of professional caliber (after all, you didn’t go to cooking school, the equivalent of music lessons), but you may have a greater appreciation for the genius who created the dish than the restaurantgoer, because you have attempted it yourself.
Out of the dense formations of endless fast food chains, Simon's novelties were to titillate the jaded restaurant goer. The Land of Look Behind -Paul Cameron Brown
a person who buys the goods or uses the services of a business, library, etc.
This is a very common business term as it has far reaching implications into other types of business, but when you're at a restaurant, patron is very commonly used to refer to the ones eating there.