She would need to keep her diet consistently low GI- she should avoid any packaged, shelved, or instant food.
As mentioned above- oatmeal (raw- not instant), granny smith apples, etc. She is the perfect candidate for the South Beach diet. The primary means that diet uses to keep weight off is low GI food. As carbs go- she just needs to avoid simple sugars period and stick with complex carbs. Any foods high in fiber that force the body to work to break it down will keep the blood sugar in check. The biggest offender is the bread. I really haven't found a "safe" bread yet. Ezekiel 4:9 bread is about as close as I have found- but I think it still raises the blood sugar.
Hope that helps.