Abstaining from meat on Friday is a discipline of the Church (and also the Eastern Orthodox) which isn't really relevant to non-Catholics ~ they have "bigger fish to fry" in terms of their final judgement, sorry for the pun, NOT. As for Catholics going into a restaurant on a Friday, it's not the owner's or server's responsibility as to what they order, and rarely would they ever know a patron's religion.
I would think that a Catholic owned restaurant would always offer suitable Friday fare on their menu and almost any restaurant will have some non flesh options. Most restaurants arould my environs that offer soup will have clam chowder on Friday's. Almost all restaurants have salads. Even bbq and steak houses will have, even if no fish, breads/rolls, salads, potatoes, rice, vegetables, beans, coleslaw. I was in an Arby's the other day and in addition to their signature roast beef and brisket sandwiches they have fish fillet sandwiches and macaroni and cheese, salads also I think. Most general restaurants always offer fish sandwiches and a fish and chips basket.