I had a himalayan who had problems. I finally discovered that stress or fish cat food triggered the problems. I had her on Hill's CD since she was not a diabetic. I gave her dry and wet cat food and avoided any fish. That usually worked for the most part but she still has some problems. She lived to be 19. I did not really think dry food was an issue but now that i know more about it, I would not give any dry food if i had to do it over again.
I have a nondiabetic one now who had stones and has infections frequently since he has had PU surgery. Along with my other one who was diagnosed as a diabetic last year, I have learned a lot more about diabetes and urinary problems. Do not give any dry food. Even though he does not need this type of food, I feed everyone a high protein canned diet (only Wellness turkey) and add plenty of water to it. I add 1/2 cup of water to two 5.5 oz cans of food at each meal. He has done pretty good except for frequent e coli infections due to the surgery he had to make the opening larger so he could pass the stones. Since he licks a lot, he gets e coli infections and has to take anitbiotics. My vet read that cranberry extract is supposed to be good for keeping e coli infections down so we started looking for one for him that did not have anything that my diabetic one could not have in case he got one. We finally found a dog treat to use. It is called Pala Tech Potassium Citrate Plus Cranberry chewable tablets. I give him a half tablet per day. We are hoping this will work.
Anyway, for your diabetic cat, I would stick to a high protein canned food and add lots of water to it. Avoid any fish (Even Fancy Feast Chicken has fish as an ingredient) and try some cranberry extract but be sure to read the label and avoid anything like glucosamine which a diabetic is not supposed to have. Do not give any dry food at all.