There is no sin on her; rather she will be rewarded, because if a person becomes sick and it is too difficult for him to fast, it is prescribed for him to break the fast, and he will be rewarded for accepting the concession (rukhsah). Allah says (interpretation of the meaning): “and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days” [al-Baqarah 2:185].
And the Prophet (blessings and peace of Allah be upon him) said: “Allaah loves His concessions to be accepted as He hates sin to be committed.” If a person avails himself of the concession out of obedience to Allah and following what He has prescribed, then he will be rewarded and there is no sin on him. So there is nothing wrong with what the sister who asked this question did by breaking the fast because of sickness. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)