I am a 60 year old diver with diabetes. Once my MD cleared me for diving I never looked back. As a diabetic diver I am in much better shape than most divers my age. I watch what I eat and drink, I work out regularly and even run a marathon or two. I am also much more aware of my limitations than many other sivers appear to be.
Please look at DAN or Dive Training magazine both have run articles on Diving with Diabetes. Any instructor who is not current with the long term and proven research should rethink their committment to their occupation..