Reference: Fataawa Ramadhaan – Volume 1, Page 292, Fatwa No.236
Fataawa libni Fowzaan, Kitaab ad-Da’wah – Volume 1, Page 153
Question: The doctor advised me not to fast because it was harmful to my health. After I had not fasted for 15 days, it became apparent to me that the doctor was not a Muslim. What should I do?
Response: It is obligatory upon you to make up [the fasts] for the days you did not fast. You made a mistake in relying upon the statement of a non-Muslim doctor, because his statement [in such affairs] is not reliable. So that which is obligatory is to consult a proficient Muslim doctor regarding this and other such Islaamic issues. And it is upon you to take this into consideration in future. And Muslim doctors are aplenty, and all Praise is due to Allaah (alone).
And Allaah is the Provider of Success.