Your girlfriend is very brave if she has flown with you five times! It is not the same as roller coasters at all. Like others suggest on this thread, it is having nothing to do except suffer your maneuvers! If you intend to stick with her in the future, invest in HER flying lessons!
I had to take quells (for motion sickness) the first ten times I went up in a glider. They should be taken half an hour before flying, and have a sick bag in reach just in case.
As I was doing more and more of the flying I needed the pills less and less...and now may still get queasy if the other guy is doing most of the flying on a long cross country flight.
So next spring, book her in on a gliding course! and then some power lessons, and after that, the more she controls the aircraft instead of you, the happier she will be.