Chronic kidney disease (CKD) with an estimated glomerular filtration rate (eGFR) of 30 milliliters per minute per 1.73 meters squared should be able to go without food and water for limited amounts of time. This partially depends on other medical illnesses that might be present including heart disease and diabetes. Typical religious fasting from sunup to sundown, should not pose a problem. Longer fasts for 24 hours might be best done is continued intake of water. I don’t think his age of 87 would be a major factor in fasting.