I'm not diabetic, but I do have issues with blood sugar, so have done a fair bit of sugar-free Disney. As others have said, make sure you carry some snacks around with you, so she can eat it in lines etc as necessary. I didn't do this at the beginning of my recent trip, and I really paid for it - after a few 'episodes', we made sure we always had some emergency snacks!
If she's OK with artificial sweeteners and other sugar-free treats, there's a whole host of things out there! A lot of the CS restaurants and quick food outlets have sugar free brownies, which are absolutely delicious! You can get sugar-free ice cream at any of the permanent ice cream places (the ones in buildings rather than on carts), I really like the sugar-free pecan ice cream; most places now have a good range of fresh fruit, I particularly like the fresh pineapple from the Dole stand in Adventureland; and there are various bags of sugar-free candy, which I think you can get under the dining plan (check this, as I've not done the
DDP before). Then there are various savory snacks such as chips (depending on how reactive she is to potato), pretzels (middling GI, but it might be OK), popcorn

, and cut vegetables (The Land pavillion in EPCOT is particularly good for fresh veg, but I don't know what counts on the DDP). I think you can also get those ready-to-go pots of cereal from hotel food courts as a snack, but this may have changed.
Just a final note - the sugar-free items aren't always listed on the menu, so don't be afraid to ask whether they have anything sugar-free. I'm used to doing this anyway, as I'm a veggie, but the CMs are always very friendly and helpful; if they don't have anything, they'll be able to point you to somewhere that does
Hope you have a great trip!
