This is my mom! She will only eat pancakes, mac&cheese, mashed potatoes, or a plain hamburger. I understand the idea of not judging what someone else eats, but the unfortunate reality is that those choices DO become a burden for other people. There are literally 3 restaurants that she will eat at - Cracker Barrel, Bob Evans, or Perkins. All of which have very mediocre (at best) food. When we go to visit for a week, it is always such a struggle to figure out dinner. Because we're not going to rotate between those 3 for a whole week! We often either end up going out to eat without her (which she makes us feel guilty about) or just get some take out to bring back to their house, so that she can microwave herself some pancakes to eat. But even then she complains that our food smells too strong. So we definitely don't dare try to cook anything substantial there. She claims that she is "allergic" to absolutely everything else, but I find that hard to believe. Unfortunately, she was raised on nothing but hamburgers (her parents owned a hamburger stand) and I strongly suspect that she just never was encouraged to eat anything else and develop a taste for it.
We have gone out of our way to make sure that our kids are well-rounded eaters. And I think they do pretty well. DS is a little more finicky than DD, but he still has a pretty impressive appreciation for good food. I am a vegetarian myself (who doesn't like mushrooms!) but I am always determined to find something that will work, wherever we end up eating. Because I refuse to limit the rest of my family to 3 restaurants, lol!