What "unpaid food"? Didn't the couple pay the restaurant $17.98?
In the absense of a policy stated otherwise, those who pay for a buffet are generally buying the privilege of having as much or as little food as they care to eat, taken in as many or as few trips as they care to make.
Although restaurants have significant fixed costs and staffing costs, the marginal profit margins on food are usually pretty huge. Even if the net prepared cost of the beef was $5.00/lb (and I doubt it would be that much, since restaurants buy meat wholesale), and even if the slices were 3oz each (which is larger than I'd expect buffet slices to be), twelve slices of beef would still only cost $11.25; I don't know how much the wife ate, but it seems likely that the two of them together cost the restaurant less than $17.98 they paid to eat there.
Further, even if the food they ate cost the restaurant more than $17.98, that would still not justify saying they were eating "unpaid food". After all, if a AAA member's car needs to be towed twice in the course of a year, would you consider the second tow "unpaid service" since the two tows together probably cost more than $52? Whether the dieting couple or the restaurant can claim to be the defrauded party is up to a court of law to decide.
I wasn't present, but I would tend to side with the couple unless there was some clear signage which indicated that the restaurant had the right to restrict the quantities of food people took. The couple likely would not have spent $17.98 to eat there if they'd realized they would not have as much meat as they wanted. The restaurant could have posted signage making its policies clear, but failed to do so.
Conjecture and speculation not based upon known facts.