The amount of coconut water depends on the patients renal health and prsence of other health problems like diabetes . If patient has only diabetes then better to restrict 100 ml of coconut water at a time if food intake is good, if not can give them additional amount based on the sugar cobtrol.
If patient has renal disease then have to check whether blood potassium is normal and coconut water is not allowed for those who have fluctuating blood potassium level.
If no other health problems are present cancer patients can have tender coconut water half an hour before meals, as generally cancer patients on treatment will have poor appetite and they may have early satiety so may feel ful with small amounts of food. Coconut water has electrolytes, calorie content is 25 kcals per 100 ml, so can be given if liked by the patient as a drink. But an underweight cancer patient will have higher nutrient and calorie requirement and it is a real challenge when they have poor appetite, nausea and early satiety as well.