If she's on medication for anxiety then I would hope that that she's already being treated, or been treated by a therapist. I would ask her, or her parents if appropriate if there are any particular strategies that you should employ in your teaching.
In response to jinxremoving I would say that it sounds to me that this rider is riding for therapy as much as to 'learn to ride' and that in this instance 'babying' her might be exactly what the OP should be doing.
Hopefully with your help she will develop confidence, and techniques to manage her anxiety that she will be able to take into other aspects of life. If she learns to ride well too that would be a secondary benefit.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.