never let them out the door! is it going to cost the restaurant? quite frankly, yes, of course it is. but remember, the restaurant made the mistake. is it going to be expensive? possibly, depending on the situation, but it will nonetheless be less expensive than if that guest left unhappy.
keep in mind it is just not just this particular guests experience at risk here. a guest that has a great experience at your restaurant will on average tell two other people. a guest that has a bad experience tells ten! make it your goal to do whatever it takes to assure the guests leave happy.
as i said above you 