hi,
i am sorry to hear about your grandfathers suffering. There are some home remedies we use for retention. These are natural diuretics and will not harm.
1. Let him start the day with coconut water and then have a glass in the evening as well.
2. Pearl Barley water also works the same way as coconut water. So maybe you can alternate.
3. He can also try the panchtatva mudra, Mudras are amazing healers and easy to do. he has to the pose for minimum of 15 mins (either hand or both hands) and should do it atleast 2- 3 times a day till problem is lessened/healed. Am pasting link here for you to read more.
love