I rarely have to police mom but sometimes she does carb out. She had to have 2 packs of pringles and learned right then and there that it is 1 pack per meal unless she had absolutely no other carbs. She knows what her limits are and will stop when she is full.
Disney deals with people with a wide variety of dietary needs. Atkins diet was a low/no carb diet and celiacs cannot have bread, for example. Every sit down restaurant can make special meals for diabetics. Diabetic meal planning goes back over half a century so chefs are well prepared to deal with diabetic guests.
You say diabetic but each diabetic is a unique person. My mother controls her diabetes by staying active as is possible and diet. Some diabetics are on a very strict diet. She is a grown woman and will do fine with Disney restaurants. It is very kind of you to make sure she is happy on the trip and avoid any problems. Definitely look up the menus which are linked in FAQs. While a diabetic many not know the portions of items on the menus I would say use what you would eat at home to guestimate how many carbs you would get from the meal. If they bring you a huge plate of fries then you eat a quarter or what ever you eat at home. Mom gets a slab of cake and takes 3 to 6 bites and that is it. On a club sandwich she ate half then picked out the meat and goodies, thus leaving the bread.