Exactly, which is why I would not charge the restaurant anything beyond the price of the meal. I see no evidence of intent to defraud, but nonetheless the restaurant failed to provide what the customer reasonably believed he was paying for. While one might reasonably quibble over whether the couple deserved a full refund for the cost of their meal, I would be inclined to give them one on the basis that (1) a restaurant is supposed to not only provide food, but attempt to provide the customers enjoyment; it appears that the restaurant failed to do the latter; (2) even if the customer deserves less than a full refund (e.g. if they ate 80% as much as they should have expected to eat, they might deserve a 20% refund) quibbling too much over less than $20 seems silly; (3) it is the restaurant's job to satisfy customers, not the customers' job to satisfy a restaurant.
I am reminded of a story told me by the man who prepared my wife's grave marker. One of his clients had his wife's grave marker destroyed by vandals about 10 years after it was installed. The client's insurance company asked him what the expected useful lifetime of a gravemarker was and he replied about 200 years. The insurance company thus decided to write the client a check for 95% of the cost of replacing the grave marker.
That decision may have saved the insurance company an immediate $50 or so, but it cost them the business of a previously-loyal custoemr. Worth it?