| You're girlfriend needs to get in at a center that specializes in diabetes.
There, they will teach her how to count carbs. She'll be given a specific amount of insulin to take per grams of carb, and be able to decide how much fast acting insulin to take. At least in an ideal world (though I imagine most people do this now).
The way you are describing doesn't sound familiar to me, and really makes no sense. I have a higher-ish bmi and take small amounts of insulin, while people with similar bmis take twice the amount I take. Or four times as much. Everyone is different.
Hopefully, she can get in a center where a nutritionist can teach her to count carbs, and a doctor and educator can help her adjust her doses.
If you say what city you are in, people may be able to recommend a place. | 
