Try anything with ginger in it - ginger ale, ginger snap cookies, etc. It's an old sailors remedy for sea-sickness and it really does work. See
here for a study on the subject, as well as some other suggestions.
Also, if she uses glasses for distance, be *sure* she uses them whilst above decks. The motion, combined with the fuzzy distance vision, is a killer. My son wears his while travelling and it has virtually eliminated his motion-induced illnesses. Also, keep her looking forward as that may also help.
I also agree with Rolf, in that if you keep busy, particularly if you are controlling the motion and direction of the vehicle (driving or piloting), it will prevent much of the sickness.