As of her diet, making sure she has a lean type protein is a big help, I was diagnosed with gestational diabetes, so that's where my past experience was from. What will probably happen diet wise is they will set a certain number of carbs per meal for her to reach. As it was explained to me, if you do a sweet do a protein. She may have to meet with a diabetes educator/dietician as well to get a diet plan in place.
Quick limits to her diet that you can begin immediately would be stopping the sodas and extra sweets if you can. If she does need a sweet follow it with a piece of string cheese or some almonds. A lot of it has to do with portion control and what you are taking in. You'd be surprised at what you think is the right amount vs what you really should have. Encourage her to drink more water during the day. Make sure if she can during school hours have a small snack in the morning and one in the afternoon (or right after school if she's not in activities) these would be something like almonds or a small bit of fruit with cheese. And have her keep active, a walk or jumping on the trampoline - anything to get that body moving. She may have to be more conscious of what she is eating if she is indeed pre-diabetic. I was always found to have a higher fasting number each morning, but the rest of the day I was in normal range. I was checking blood sugar 4X a day and was able to control mine with diet and exercise. Since having GD I now have to be checked yearly for diabetes. So far, my A1C has remained good and in the normal range. I just have to continue remaining mindful of what goes in my mouth, exercising and taking the weight off is good help.
I don't know if you looked here:
http://www.mayoclinic.org/diseases-conditions/diabetes/basics/tests-diagnosis/con-20033091
Good luck mom! Another place you may look is diabetes.org for some help.