If she's anything like Tobey she will get there in the end! He used to vomit on a 5 minute car journey but last summer he travelled all the way back from Dorset (about 6 hours in total) without a single problem!!! We did give him some scullcap and valerian tablets to start off with and they seemed to do the trick (another useful tip from a PTer I seem to recall)

http://www.dorwest.co.uk/herbal_medicines.htm