it's probably the battery. though starting the car is good practice, the engine isn't running long enough to charge the battery (unless she starts it and runs it for a few mins) and the battery has probably lost charge.

it's probably not dead, but weak, which is why the lights and radio still come on. it's just not enough to run the starter motor, hence the clicking. though, it should be noticeable that the batt is weak because the lights will come on weakly and the beeping when you put your keys in the ignition would sound weird.

try jumping it, and once it starts, drive it around for a good 30 minutes.