IMHO, they would do well to have several entrees of which it would be very easy for them to make vegetarian versions.
For example: If they have a fetuccine alfredo with chicken, it would be very easy for them to substitute, say, broccoli for the chicken, if they cook the chicken separately, but rough for them to leave it out if they cook the chicken in the sauce.
Instead of having to come up with a whole vegetarian menu, at least I personally, as a vegetarian, would be happy if on their menus they overtly offer to render a vegetarian version of as many of their entrees as they can.
Certainly, they should have garden burger patties or the equivalent, available to substitute for burgers or cold cuts in any of their sandwiches, and vegetarian sausages available to substitute in any breakfasts, for meat sausages or for bacon.