I used to be a food & beverage director for a high-volume entertainment business.
From my perspective and experience, the cast member on the front line is the least guilty party in this exchange. The manager
AND customer however both errored in this case. From a
loss prevention standpoint, you can't allow your employees to offer discounts or give away items. If you do so, then you will have to deal with the theft of goods.
In a situation like this, the employee should simply apologize and inform the customer that the item is unfortunately not available at this time.
The customer errored since they should reject a serving like this and never pay for it. If the item that is being served does not meet your expectation or what is advertised then walk away or escalate to a manager.
Now if managers are authorized to give something away then the manager in this situation could've said we're unfortunately out of this product, but in this instance, I'll allow you to have this serving for free. However, where I worked it was our policy that we were never to serve anything that did not meet our standards, so in this case, we would apologize and suggest something else. If you pushed the issue my managers had the discretion to fix the situation which may have included offering something for free. We never discounted, because it even encourages abuse and theft amongst management when given this power.
We also never gave away our errors. I know many food establishments will just let you have it when they make something you didn't order, but that again sets up a situation where an employee may purposely do that so they can give it away, often to someone they know.
It's always a disappointment when something you want isn't available, but it is a recurring thing in life.