What foods are good for a diabetic dog?

Although I have diabetes myself and I know what foods to eat and how much insulin to take according to how much food I eat. My friends dog has been diagnosed with diabetes and we're not sure what foods are best, and the quantities too. The vet tells her, that the dog MUST have a certain food (which they conveniently sell), yet Jess isn't impressed with this food and is refusing to eat it. Any ideas please?