speaking from experience, it is really hard to sew if the foot pedal is on the table and you are using one hand to push it. I never knew how much I used two hands sewing until I had to do this on a vintage machine with an 18" cord. I agree with Joe- a way to operate the foot pedal by leaning on it might help.
On my Bernina with the button operation, you can't control the speed without taking your hands off your work. I would find that hard to do.
maybe a modified knee pedal she could lean onto?