I agree with Sarah at work.

This is a little vague, but if you are very lucky, your student will be studying piano so she can duplicate a sound she's heard. Ask her what she wants to play and take her answer seriously. Make that sound your shared goal and make sure she can see that the two of you are moving in that direction with your lessons. For example, if she wants to sound like Elton John, work out a simple arrangement of "Candle in the Wind" or whatever and start working on it.

If you are not lucky, your student will be studying piano for some other reason. Unfortunately, you, as the teacher, have minimal (actually no) control over your student's daily life. It is really up to the parents to get the job done, so you need to get the parents into the loop. If they're not willing to help you and do what it takes, there's no hope.