Sometimes it just happens, i have never been seasick carsick or airsick until i did an intensive hour "under the hood" which involved some recoveries from unusual angles in instrument conditions.
I was being vectored to land and felt really ill, had all the vents open (which takes a bit fitting into the scan.. ASI, vent, HI, vent ALT, bag) told the instructor, he took over, i didn't blow chunks but i did sit on the wing for a bit and have some negotiations with stomach.
The day before doing the same amount of hours i'd actually landed the airplane (with some help on the round out ) totally under the hood, no issues.
If it happens all the time the guy needs to do something about it medically or consider giving it up, lets face it if you going to puke on every 2nd flight why bother.
If its a one off its just really bad luck... i lost about 5kg in sweat on my Practical! poor bastard