I understand your daughter is probably quite stressed about this. To hasten recovery, I would suggest not talking (whispering also causes strain on the vocal cords), using a cool mist humidifier, and drink lots of fluids. Using steam (hanging head over steam bowl a couple of times a day), and breathing through the nose instead of mouth, will also help keep the vocal cords moist.
I am hoping she doesn't smoke; if she does, this will further irritate her cords.
If she is still having problems though when it is time for her performance, she should try to take into consideration that it may be better to forego a short term performance than risk longer term damage.
I hope she recovers quickly. Good luck to her.
JMK MD
Thank you (and you're right, she does not smoke)!