Though it is not clear what causes motion sickness to children, most believe in the superstition that those affected are just not used to travelling by airconditioned vehicles. Distracting the child may help in preventing this. Encourage your friend to make her daughter lurk outside the car, listen to music, sing songs, or tell stories to do this. Avoiding the use of strong car fresheners may lessen the risk of barfing too.
My sister used to be like this before. What my dad did was to stock up on candies in his car, so maybe you can do it, too. Bring some candies; mentholated can be even more helpful because of the smell. Also make sure to bring some plastic bags just in case she can't control her barf and throws up.