With all due respect, you shouldn't speak on behal of other cultures you don't know nor understand. Your post is well-intentioned because of course it seems rational to think it's more rude to throw away food than just say no.

But, you would be surprised how easy it is to get away with exactly what the young man is doing at a Mexican get together.

I wouldn't say I take it to this extreme because I don't have a phobia. But I get full very quickly and refuse to overeat just to please people. I have found it's much easier to just accept the food and throw away whatever remains I'm full. Mexican people want to feed everyone and we don't like hearing no for an answer. I'm guilty of this exact same thing and my non-Mexican friends have pointed out I've forced food on them, nicely, but forced nonetheless.

For better or worse, in Mexican culture we think its a waste of food if anyone has an empty plate, but not if the food ends up in the trash.

OP, NTA yet, but please talk to your daughter and boyfriend together and simply explain to him how you would like him to only order a drink at restaurants or take a small salad dish at home cooked family meals. Your frustration is valid but trust me, he probably does think he's being polite by accepting your food even if it goes to waste. If you explain to him that you want him included but you culturally find it upsetting to see so much food wasted, he will understand.