Yep my youngest daughter when she was around 14 started getting crook minutes/hours after eating (always ended up over a dunny vomiting violently)... after a bit of testing and trialling and Dr's visits, we delved into the gluten free thing, this made improvements but still being crook... then tried removing dairy..... no more being sick, lost weight due to limited to mostly raw non processed foods and generally healthier and happier and has accepted now a couple of years later that this is now just the way it is..... So like plenty of others who know, shopping for both GF & DF foods is a challenge, but not impossible.
Earlier on after going GF/DF for a month or two we thought we'd test the GF theory, slipped some wheat type food into a meal (she knew it was in there, and it was her that wanted to try it, think it was crumbed steak) and guess what, about 2hrs later heaving her guts up...... poor bugger.
She's now cruising along with the whole thing though, occasionally gives me a cranky look when she see's me hooking into a fresh slice of normal bread, or a pie etc. As for sleeping at mates places, yep, she packs her food and doesn't really think much of any one else's opinion of that.
As for the people who are jumping to the "some people breeding sick specimens of kids" idea.... I can't type here what I think of that assumption
...... we have 4 kids, all been raised the same, all grew up out bush with plenty of knock-a-bout kid upbringing.. plenty of time outdoors, eating whatever we cooked up.. no fuss type kids and no helicopter type parenting, the older 3 showed no issues ever and still today with foods.
I honestly believe these types of allergies have more to do with the modifications that have taken place over the years that have changed very natural foods into something it was never meant to be..... changes made to make them faster growing and yielding higher values for production purposes, I'm no expert on farming but have seen enough doco's and articles on this type of thing to conclude there has to be something in it.