Some genotypes cause more gastritis (stomach inflammation) more than others. This can cause nausea.

Allergy. It is always possible to be allergic to something, but usually a person has to develop an exposure to the toxin before they develop the allergy.

Some medications do not work right in some folks, as an example, I used to take a medication to prevent angina and heart palpitations. It did the exact opposite and caused those things to be much worse.

So, it could just be your friends bio-diversity.