I have great success with reducing cramping in clients with two things.
One, stretching the affected muscle frequently. This has been especially helpful with calf cramps.
Two, drinking a sport recovery drink.
For clients that experience cramping at night, they tell me keeping a sports drink on their night stand for this is helpful. And gentle stretching of the muscles affected right before bed or upon being woken by a cramp also helps. The gentle stretching also can be helpful in endurance events. The time lost to stretching beats crumpling in a heap from pain.
There are some other techniques that you can learn, but I would not be comfortable trying to explain them without including a hands on component with actual cramping athletes/clients.
I would like to add alignment and proper sitting technique while driving to all the most excellent ideas presented already.
Sara, I agree with you, that myofascial release techniques would probably be the best way to help alleviate this problem, along with increased water consumption. I would suggest that she use a foam roller prior to going on a longer road trip. I sometimes have leg cramps and will use a tennis ball if I don’t have a foam roller available.