Answered by Shaykh Faraz Rabbani
Question: My sister-in-law has been diagnosed with diabetes and advised by her doctor not to fast. Does she have to fast? How much expiation does she have to pay if she cannot?
Answer: I pray this finds you in the best of health and spirits.
1. It is not an “automatic” that every diabetic person is unable to fast. If you’re able to, encourage your sister-in-law to discuss her specific situation with a discerning scholar. Many doctors (fearing medical liability) err on the side of excessive caution in such matters.
2. If it is established that she is presently unable to fast, then there is no expiatory payment (fidya) for her to give, unless she is both elderly and the condition is not likely to ever improve.
[ref: Shurunbulali, Maraqi al-Falah]
And Allah alone gives success.
(Originally answered on the SeekersGuidance Ramadan Course Forum)