By Diabetic coma
do you mean she's comatose because of very high sugars or because of severe episode of hypoglycemia
(low sugar)? This question of yours needs more of a neurologist's opinion. But knowing from what you have described here, prologed dehydration
of the nervous system due to very high sugars is the reason for coma in case of very very high sugars. Or if she has had a sever hypoglycemic episode then deprivation of the brain from glucose
for very long time is the reason for coma. When your doctor says she might not improve at all must be on the basis of her current condition- not only the sugar or intact brain tissues seen in the CAT scan. If she's lost consciousness for more than 12 hours before she was subjected to the intensive treatment, by that time because of temporary dysfunction of brain centres, other organs' functions get affected and she might have failed her kidneys, lungs or heart to some extent by then. So multi organ failure following prolonged state of unconsciousness
can be the reason that your doctor has made that statement to you. Take care. Wish something magical for your mother and recovery. all the best.