I'm abso-fucking-lutely rotted at the moment, and I'm out for bear. If I see him on my own travels today I will rip into him so hard he won't know what the hell hit him.
First parsed as "out for beer". Which might not be a bad idea.
Good luck with this guy. He probably thought his IBS was a get-out-of-jail-free card, until he ran into a superior (you) who also has it but deals with it appropriately so it doesn't interfere with life. There's some pwnage there if you look closely.
Regarding "trigger foods": Is he ignorant, or just doesn't give a
damn? If the former, he can be educated; if the latter, then there's not a hell of a lot you can do for him, or with him either.
(I had a great-aunt and -uncle who were both diabetic. She took care of herself; he ate what he damn well pleased, took his insulin when he felt like it, etc. They both lived about the same length of time, but her quality of life was way better than his, e.g. she died with both her feet still attached, etc.)