With conscientious carb counting, there's not a single food that's off limits for diabetics. I chose not to eat fruit (or many carbs) for years because the more of those foods I consume, the more insulin I require. For me, the less insulin I use, the more likely I am to be able to control my weight. Plus, many of the foods that require the most insulin (simple carbs) just aren't that good for anyone.
I am using fruit in my smoothies but only about 30%. If I wanted to use more I'd just use a little more insulin. Diabetes is all about balance; food, exercise, medication.
If your aunt has a good understanding of her insulin use, she could add smoothies to her diet. If she's a type 2 diabetic on medication, she could still add them but may need to subtract carbs elsewhere or follow a smoothie with some exercise. If she is type 2 and not well-controlled on oral meds, I'd encourage her to consider insulin, which is a much more natural approach to helping our bodies work normally. Insulin is not a punishment, it's a great tool!