Is your mum being treated in any way for the nocturnal seizures?
How long do they last and how frequent are they?
I'm not sure where you are, but i'm in canada and most often here you have to be seizure free for 1 year before you can drive again. Is your mum afraid of having a seizure when she is in the car? Does she get any kind of warning or aura? Have you expressed your concerns to her?
I realize that it probably sounds like i'm drilling you..Sorry...I just need more info before I feel I can really comment and i'd like to offer you support if I can.
Kathy