It sounds like they were both enjoying the opportunity to be mad.
The one thing you might have done (but i wouldn't have been fast enough on my feet to think of it!) is, when she said, the first time, "Oh, gross, I can't eat this!" is to say, "Oh, goodness, let's call the waiter over and discuss it with them!"
It might remind her that her audience is wider, and it has a bit of the "what are you going to do about it?" tactic to it (that's a great phrase to use for someone who's whining about their weight yet again).
Another good reflex to cultivate is that when other people get loud, you get soft. When they get rude, you get ultra-considerate. Sort of, role-model for them how it *ought* to go.