Different perspective - I was underweight at 20 and treated for an ED, I was overweight at 29 and still had the same impulsive sugar hits I did at 13.
It took serious health concerns for me to actually kick my bad habits, and even that is two years on and I'm only just focusing on those sugar binges.
If she has AdHD it could be several things, mood crashes at a certain time of day, sensory seeking (I love sweets because I enjoy the chewiness of them) or sometimes a craving for something crunchy and I reach for biscuits, it could be a dopamine hit from the sugar, it could be she feels really iffy and almost ill and needs the sugar hit to feel better.
Until you know the underlying cause the food noise will win.
I've only just started beating it by working out how to tackle my sensory seeking, I chew gum now instead of sweets, I buy a particular low fat low calorie type of crisps rather than sugary biscuits, I drink squash through the whole day rather than one sugary drink at lunch that kind of thing.
I don't really know how I would answer the direct question about looking fat though, it's hard when your on the spot and my face will say more than my mouth without meaning to! But I'd have to sit her down and say you obviously don't feel happy with your body as you keep mentioning it, your clothes aren't comfortable and your not happy so let's get on the scales but you look at the number not her, you can tell her if she wants to know, all you need to do is say doing some exercise classes together and tracking our calories together would be fun, I'll find some fun recipes and we can use the same app.
Also there's an app that scans your body and takes your measurements, she'd be able to see for herself without it actually mentioning weight as she can scan once a week for free and it'll tell you the differences.