Overweight people are more resistant to their own insulin and need to produce more insulin to effectively metabolize glucose in body cells. If your friend were to lose weight, her insulin resistance would improve. Even if a person limits their sugar intake though, excess calories can contribute to an above-normal blood glucose levels.
A low-fat eating plan is recommended as part of a diabetic nutrition therapy with around 25% to 30% of your calories from fat, 50% to 60% carbohydrate and 10% to 20% protein. I am wondering if her doctor has measured her blood fats (cholesterol and triglycerides) and whether they are within normal ranges?
Excess weight and foods high in fat intake are risk factors for heart disease and many types of cancer. It is up to your friend to choose to lose weight and control her fat intake. Even a 10% loss of body weight can make big improvements in blood glucose levels. You can be supportive of her positive efforts. Nagging her about her food habits will produce negative results. Why not create opportunities for you both to get some exercise together?