As a former mariner, I'm not unfamiliar with that problem in others.
Without going into the physiological causes and effects, I'll just say -
The golden rule is...
GIVE THEM SOMETHING USEFUL TO DO.
Motion sickness, particularly seasickness and that suffered by aerobatics passengers, creates a feeling of hopelessness in the sufferer.
At its worst, they just want to curl up and die.
If the person is made to feel that his/her active participation is essential to the safe working of the craft, things will usually change for the better.
Having them regularly refer to the horizon (if available) and reporting any sightings (of whatever) also helps.
Even create a "crisis" if you need to.
The important thing is to get them ACTIVELY involved - they'll soon come to.
Uttering soothing, sympathetic words is the worst thing you can do (aside from leaving them to wallow in their own misery, of course).
Hope that's some help - it usually works for me, anyway.