Just like every other muscle, to best work your core you need to hit it from all angles, doing all different types of movements.
I prefer medicine ball sit-ups on a decline bench, but I only am able to do those when I have a partner.
As far as your mother's headaches go, I really have no idea. It could be that her body just isn't used to it and that they'll go away with time.. or it could be that shes going from 0-60 right away. Perhaps start her off with some intense walking on an incline before going right into the treadmills, bikes and stairsteppers.