I think it is highly unlikely. Potatoes and Corn are vegetables and corn is in 100s of commerical products so it is nearly impossible to avoid.
A vegetable is part of a plant while a fruit is the ovary of the plant so they are both plant products and thus contain the same chemcials.
I suspect the diagnosis came from his mother, who obviously found a unique way to eliminate this issue from her dinner table every night.
That is an uninformed response. You do not know what is going on with that man's body. Likely, this man does not know why he is allergic to the vegetables. I began developing food allergies in my twenties, and the list of offending foods just kept growing until almost all fruits, vegetables and nuts were included. I ate fresh fruit, fresh vegetables, and raw nuts constantly as a child and then one by one these things have caused me to 1) go into anaphylaxis, 2) vomit, or break out in hives. The reaction depends on the offending food. I am allergic to soy, but for some reason have no problem with dairy. I can't eat carrots, but I can eat potatoes, corn and wheat. I would kick you in the shin- twice- for an apple, kiwi, or mango right this minute. When discussing my nutrition, my allergist, who spent many years working almost exclusively with food allergies, told me to be thankful that I was not allergic to meat. The doctors are still running tests because I seem to have elevated numbers of a cell that indicates your proclivity to allergies. So don't assume you know why he does not eat vegetables.
it's not that it's unheard of to be allergic to a specific vegetable - it's just that one guy's claim that he's allergic to ALL vegetables because 'someone' told him 40 years ago is a bit far fetched... especially since it's not followed up by stories of visits to allergists, food challenges, epi pens, bloodwork, etc etc... the way you have described your experience, Luke.