Well, an unfortunate incident! If the cast has a waterproof lining then the water should seep away. Also, there are air pores in fiberglass casts. So using a hair drier on cool setting should help. You can also use a portable fan. If you feel that the skin inside is still wet then show this to her doctor. If the skin remains wet for too long, it can cause infection and the skin may loose its texture and tone. You can wait the night out though!
Hope this helps. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.