I was an overweight teenager and kid. The gift of low-carb is the greatest thing you could give her. My biggest regret in life is that I didn't learn about low-carb sooner than I did. Exercise is a scary thought when you're overweight. She doesn't have to do much for exercise early on so she shouldn't be intimidated. It wasn't until I was 22 that I really started educating myself about diet, exercise and making myself all-around healthier.
I'm not sure of this girl's personality, so she may take it either way: as an insult or a helping hand. What do you have to lose as long as you have her mother's permission? Being given so much conflicting advice is the worst thing she could receive. At least you've got the proof to back yourself up! RESULTS SPEAK LOUDER THAN WORDS. Let's just hope she sees this. She is so young that she obviously isn't knowledgable about nutrition, insulin or long-term health risks she may face later in life.

Please, write her this letter. Or could you talk to her in person? Either way, get your story across to her. She has her whole life ahead of her, she could go so far!! My heart really goes out to her, and you, for being such a kind friend to offer her this much needed help. Good luck to you both!