The son was a rude dick. Full stop. He sounds like a bully that was using whatever was on the table to humiliate and belitter his mother under the guise of concern. With all that food, there was obviously something the woman could eat, come on!

I don't think you want to know how I would have handled the situation, but since you asked, I'd have told him to shut the fuck up and if he wanted I'd go grab her a carrot and a glass of fucking water to shut him up. There's no need for that kind of rudeness. Okay, they were remiss in failing to mention her issues before, but there was no reason this guy (who sorry, sounds like he's over-compensating for some sort of shortcoming of his own), not to just quietly pull you or your mom aside and explain, albeit late, that his mother's food restrictions required something other than what was on the table. I'm pretty sure your mom would have quickly accomodated. Fuck him.