I went on a medically-supervised, very low calorie and low carb (keto) diet after being diagnosed with diabetes. A few months and about 50lbs in, I had no signs of insulin resistance whatsoever, my A1C was down to completely normal values, and I was able to discontinue my medication entirely. That diagnosis, as hard as it was to hear, was one of the absolute best things to ever happen to me.
It's heartbreaking.
And nothing I can do will change her diet.