I'm no expert, but I have asked here also previously. Am going through the same somewhat. Seems like after 40 minutes, I sometimes start to feel it. Excellent advice here helped. Mainly what muddy00, korben88, FlySince9, and others here mention.
I can't get ginger snaps, or chewing gum, but the drugstores here carry ginger tablets. It seemed to help. In a pinch, real ginger.
I read, haven't tried, putting pressure on just above the wrist between the two bones can help. Hard to fly that way but I mean if it started getting bad and the CFI took control she could keep it in mind.
I hadn't caught on to the light meal thing. Definitely will try that.
Also as noted, it gets a lot worse if too hot or stuffy in the cockpit. Get some fresh air on you (even if it makes a little noise in the mic) and make sure you have a bag with you (her I mean)
Some of it seems to be in the mind too. I feel worrying too much about it can cause one to go looking for that feeling and can make one over monitor oneself, and cause it. I find it best to think, "it's no big deal. Just enjoy the flying!" And not try to feel if it is coming on.
One last thing which was mentioned a little. I just read a pilot mention studies done that show if one feels sick and lands before getting to the point of throwing up, their tolerance will be better the next time. If on the other hand they don't make it and end up throwing up, they will experience lower tolerance next time they fly.
Good luck to her. So far I'm determined to fly, and just hope things will improve. I am encouraged by those here that mention getting over it after time and experience.