i am afraid if the pup is diabetic the prescrip food would be the way to go. i don't know if a grain free food might work...maybe wellness core. maybe see a holistic vet to see if going that route would work.
much like human diabetics a high protein low carb diet gets very good results and makes it easier to control blood glucose. evo makes a nice one, so does origen. If your friend is interested this might also be the time to consider a raw diet. But whatever the decision get a meter learn to use it and test, test again and test some more. Keep a record and you can see patterns emerge and know what has to be done to maintain a good balance between diet and insulin