We know the fur ball well here... and did try the hairball formula for a while with our kitties. Don't know what caused the diabetes... but we have always fed dry, and years ago I am convinced that a change in dry food caused a diabetic spike in our older undiagnosed diabetic kitty who did not make it thru. I am glad to know at least a tiny bit more today.
Brushing helps a ton, but adding a little oil to the food (veg or olive) can also help. There's a product called Petromalt? out there... but I am sure it has malt syrup. It would be something you would use only here and there if you brushed regularly... but I just don't know about that with a diabetic kitty. I have also heard that mineral oil can help, but can cause stomach upset... so prob. not!
Wondering if smaller feedings spaced out might help? Do you know if it's fur that causes the upset? Our sensitive stomach kitty that is not diabetic is doing fantastic on Fancy Feast Classic Beef. We also add a little bit of water to the wet, as kitties who eat wet food sometimes drink less water. Since switching to wet, we have only had two fur balls with 4 groom crazy cats... it's been over two weeks and this is definitely a record. Lulu is long haired, and Hunter is the one with the sensitive stomach... we were averaging 4 or more fur balls a week. I don't know if this has anything to do with dry food or what.