Perhaps the difference between your friend and I might be explained as follows.
Sometimes we think that an unsafe food acts directly on our GI system but frequently this is not the case. It may act indirectly as an energy source for intestinal bacteria that can convert it into a GI trigger.
Fibres, fats and oils that are not digested or absorbed in the stomach and small intestine are food sources for the anaerobic bacteria in the large bowel. The bacteria ferment the carbohydrates and fats into gases, short chained fatty acids and other by-products some of which can act as GI motility stimulants or depressants. For example, recently it has been shown that methane gas produced by some bacteria can act as a GI motility depressant.
The ecology of the bowel bacteria is unique to each individual and it can be modified by diseases, antibiotics and even the things we eat (e.g. prebiotics and probiotics).
The uniqueness of the ecology of bacteria flora in each individual may explain why a food can make some people C and some people D while others don't have a problem with it.
I have never stated my age You can find it in the title of a song on Beatle's Sgt. Pepper's Lonely Hearts Club Band album
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS
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You can find it in the title of a song on Beatle's Sgt. Pepper's Lonely Hearts Club Band album 
