Diabetes limits the metabolic rates in our body. Now we require an optimum metabolic rate in our body for the proper absorption of nutrients, carbohydrates, proteins etc. Coffee can also lower the metabolic rate in our body.
So if a diabetic who already has a low metabolic rate, drinks a lot of coffee it would lower his/her metabolic rates significantly. So it is ideal for a diabetic to control their coffee use. In fact, it would be better for them to switch to green tea from coffee since green tea is considered very beneficial to a diabetic.