True, if she really wants to quite then that is her decision, and hopefully she will come back to it at some point.
The best I can suggest is to just talk to her about why she wants to quit. Listen to her, and just let her know that it is her decision but that you are sorry because you think she has real potential.
In the end, however, you really don’t know if this is just a rebellion thing or if she really just doesn’t enjoy it. Just because she may be rebelling against some things in her life now that doesn’t mean that every decision she makes is just a matter of rebellion. She may simply not enjoy playing and so wants to explore other things.
In the end it is her decision. You can give her encouragement, and you can give her an ear to listen, but what she really will need is respect for her decisions.