My significant other is in a similar situation. A reasonable amount of wheat-based baked goods (a burger bun, a slice of bread, a slice of pizza) will almost invariably set her off (causing both skin and intestinal distress). And if it sets her off once, it'll
always set her off. Small amounts of seemingly similar wheat-containing foods (eg, a bite of someone else's sandwich) will generally not. Non-gluten containing foods seem to be fine...but even when she's great about avoiding gluten, she'll have occasional outbreaks, strongly suggesting it's not (just? only?) gluten.
Her doctor has labelled it as gluten sensitivity, but we're well aware that it's not really clear she has any such thing, or if such as thing even exists. The FODMAP/fructan stuff sounds promising, but one common source of FODMAPs is garlic, and she adores garlic and eats it with everything in vast amounts which 1) strongly suggests that at least some FODMAPs are fine for her and 2) makes her extremely reluctant to try a FODMAP free diet.
The whole this is baffling. She clearly doesn't have coeliac disease, small amounts of gluten are (normally) fine, large amounts of things containing gluten are not fine, very similar things without gluten are fine, but something other than gluten that is consumed even when gluten is being avoided is not fine, at least some high-fructan/FODMAP foods are fine...I feel like I'm missing a piece of the puzzle. My current pet theory is that it's a reaction to at least two different substances, but what?
For now she just avoids gluten and (mostly) avoids outbreaks. It's confusing but it's an easy coping strategy.