How Much Glucose Can A Person Take In A Day ?

Hi, My friend is 23 years old, male. Last week he underwent an operation in which some dead mass was found in his abdomen and it had to be removed. He has become weak after this and doctors have also advised him to take rest and not to lift heavy stuff. I give him Glucon D throughout the day to keep his energy levels high. Apart from that, I also give him sweet lime/water melon juice once or twice a day and some fresh fruits too along with the regular food. I know that having too much of glucose will increase glucose levels in the blood . So, I wanted to ask, how much glucose can he take in a day so that it does not effect his health? What else can I add to his diet so that he recovers fast?