Why are the keys on the left side of my sister's CASIO keyboard not working?

There is usually a sliding switch with settings for whether the left-hand keys work as normal, play chords automatically, or use a different voice from the right-hand keys. Usually one of the settings will say "Fingered" or something close to it. Try to find that switch and see if setting it to a different position solves the problem. Sometimes it is part of the power switch.