What is the ruling on fasting for someone afflicted with chronic diseases such as diabetes and heart conditions, and is their fast valid despite severe hardship, or is fasting obligatory, or is breaking the fast permissible?

For a sick person, there are situations concerning fasting:

First: If fasting does not affect them, such as a mild cold, then fasting is obligatory for them, and it is not permissible for them to break it. Second: If fasting causes them hardship but does not harm them, then fasting is disliked for them, and breaking the fast is sunnah for them. Third: If fasting causes them hardship and harms them, such as kidney and diabetes patients, then fasting is forbidden for them.

Whoever is afflicted with a chronic illness that is harmed by fasting must break their fast, and fasting is forbidden for them.

Regarding making up the missed fasts (qada'): Whoever's illness is expected to be cured must make up the missed fasts after recovery. Whoever's illness is not expected to be cured must pay an expiation (fidyah) by feeding one poor person for each day.