Is fasting obligatory for a Muslim patient who suffers from kidney stones and is required to drink no less than 3000 ml of water daily?

If a person is ill with kidney disease or another ailment, and fears that fasting would cause illness, or worsen it, or delay recovery, then he may break his fast and is obligated to make up for it. However, if trustworthy doctors determine that the disease is chronic and beyond hope of recovery, then he is not obligated to make up for it; rather, he must feed a poor person for each day.