It might not be his/her fault. Some people with Aspergers have a problem with certain food textures. It might even be a mild form of Aspergers that hasn't been diagnosed.
I myself have a problem with the "crunchiness" of vegetables and can't eat them. I've tried to overcome it but my throat closes up when I feel the texture and know that I'm eating it.
So I can't eat vegetables. I joke around that I'm a meatarian since I eat everything but vegetables (defined as exclusive, since vegetarians are people who eat anything but meat, also exclusive). But it's not really a joke.
As a consequence I've had to make sure I'm eating healthy with this extra restriction. I used to do what they did and just eat what I like and take a supplement plus 1000mg of vitamin c. But I've changed and now eat lots of fruit and no snack food. And I've decided not to take the supplements anymore.
Supplement also tend not to be absorbed well into the body so their use is questionable. But is it better they take a vitamin supplement vs not taking one on the the same diet?