When your crank arm falls off like that is serves as an indicator that your bike is no longer safe to ride and that you need a new one.

If you are the daring type you can check the spindle and crank arm to make sure they are not rounded out and of they look good can reinstall the crank arm and torque it to spec.

If it comes off again you will need to get a new crank arm... the steel spindle will probably be ok.

Is you aunt single ?