She is insulin dependant. Thanks I know several people that fast, and that are 'not supposed to'.
Diabetics have to maintain their 'carb count' to control their sugar levels.
Hopefully, she will use a carb substitute while fasting. The Bible speaks highly about the benefit of fasting, but if she thinks she can sustain her health she is not being responsible. God wants us to fast 'responsibly'. Many health concerns do not allow fasting, but many people mis-understand the 'step of faith' that she is about to take. Religions that practice that type of faith are called cults.