some older models have a "outlet" with the foot pedal. One is for the motor and one for the light. If you plug the motor in the light outlet, it was full power all the time - takes off like a jet. If you plug it into the motor outlet, it only works with the foot pedal. However if the foot pedal is bad, then the motor won't get power, so it won't go at all.
Sounds like her motor is fine, and her pedal is shot.