There is no clean one-liner you can give her. And as I understand someone who is eating an overall high fat diet and who is also diabetic, potatoes and white bread likely does comparatively higher blood sugar. So such people probably won't be convinced otherwise as they believe what their blood sugar test results tell them and likely won't ever go to low fat diet and will likely believe carb demonizing, especially simple carbs. Such people often think they need more high fat foods which are actually really the worst for diabetes but don't show up in blood sugar test results when you are an eating an overall very high fat diet anyway. Only simple carbs and carb heavy meals show up as noticeable blips spiking blood sugar over acceptable levels or on that borderline to such people who are diabetic and eating an overall high fat diet.
Some said to recommend Forks Over Knives, but Neal Barnard has produced a DVD just on diabetes, "Tackling Diabetes with Dr. Neal Barnard DVD":
http://pcrm1.ultracartstore.com/shop/DVDs/HEAL-206.html