That's ridiculous! Clearly her doctor has never heard of "lean diabetes." However, if her BG is going way up and then crashing multiple times each day, it's possible that an A1C might show something close to "normal" because it averages out the highs and lows over something like a 3 month period.

Perhaps you could get her a blood glucose meter and enough strips to test her blood sugar 4 or 5 times a day over a few days. If she tests within 15 to 20 minutes after eating candy/pizza or whatever and then again when she's feeling woozy or hangry or shaky with a blood sugar crash, she'll probably see numbers ranging everywhere from 50s up to well over 200. Take all those numbers, along with time taken and how long it had been since her last meal, to her doctor. If he STILL won't investigate diabetes, then get her to a different doctor cause the one she has now clearly has no business practicing medicine!