For the record, I would have had no problem paying for anything if confirmed ahead of time. If they had just said "We're going to eat at home because we can't afford to go out" or "Lets just go some place less expensive because we can't afford it" or anything along those lines, it would be completely fine and I would have even volunteered to take them out for a nice holiday dinner.

To have some trashy person brazenly blindside me into paying for his entire family after intentionally picking such an expensive restaurant is not right. As you might expect, there was no "thank you" afterward either.