Honestly, anyone who has very unusual dietary needs or habits is going to have a challenging time eating at a cafeteria or even at most restaurants.
I was a vegetarian for 6 years in the 90s, my mom (who lives with us and has Alzheimer’s, so I’m responsible for her diet) is gluten-free (although not allergic), and I have a very unusual (and severe) allergy (poultry, including trace amounts of chicken broth), so I’m not unfamiliar with or unsympathetic to dealing with dietary restrictions.
With my severe allergy, I’ve simply learned to never order certain categories of foods, not eat out where the staff aren’t completely fluent in English, and I always keep antihistamines and an EpiPen handy . . . Most restaurants have chicken in over half their dishes. Sucks for me, lol.
For a vegan or someone with other severe food restrictions, I think that realistically, they’ll need to be comfortable finding alternate food sources, always keeping their own stash of food available, etc. For my mom, I always bring snacks, crackers, energy bars, etc when we are going places that might have finger foods out.
So, anyway, if my kid were allergic or had a health requirement that led them to have a very restrictive diet, then I’d help them negotiate with the administration to get out of required meal plans and help them get in housing where they could cook for themselves – to ensure their health and safety. (Might even be fine in a suite if on the same floor as the dorm kitchen . . . and maybe have their own small fridge in their own room in addition to the communal fridge.)
But, if my kid were simply living a lifestyle preference (as I did while a vegetarian), then I’d be sympathetic, ship them snack foods, make tofu-turkeys at holidays . . . but not really get bent out of shape about their lack of choices and options. It is a simple reality that they’ll have very limited options - - - because they choose to take many/most options off the table. So be it. They won’t starve. They’ll be dealing with this lack of options on and on. That’s OK, as it is just the adult consequence of the adult choice they’ve made about what to eat. They’re dealing with it every time they go out to eat or get pizza with friends, etc. I’m sure they’re not too bent out of shape about it. (I know I wasn’t when I was a vegetarian. I just ate the bread, the salad bar, and baked potatoes. A lot! There weren’t many vegetarian options at restaurants in the 90s, lol.)