There is no evidence that the service was poor quality. There is more evidence that the group of people were cheap fucks who wanted to not pay the fee. If there was a problem with the service, they would've spoken to the manager about it during the meal. Not after they looked at the bill and saw the $$$ where they want to be cheap fuckers. That group is a lot more common in restaurants, they are usually loud and obnoxious and at the end of the dinner, they weasel out with whatever they can to not pay. It's also typical that they eat the entire meal, then tell the waiter they want it replaced since it was not prepared well.
The evidence points to the restaurant being right and customers being wrong.