Turning green
Motion sickness I'm sure is as a result of the confusion of the senses. What the eyes see, the balance sensers within the ear sense and the feelings the body receives from 'g' become at odds with each other. Nothing adds up and confusion reigns, the brain, as if to say it must be what you have eaten, orders the stomach to get rid. A swell, in my experience, is worse than turbulance in so many cases.
The sufferer must not focus on one thing nor sit motionless. Get them to move about in their seat and continously move their head and body to break the cycle.
Having not eaten and being low in sugar is often the cause and of course so is tiredness. I always check whether a Trial flight has eaten recently and if not get them to have a Mars Bar or similar, particularly before an aerobatic trip.
Seems to work for me.