the 'traditional' approach to managing diabetes has been to tell people to eat whatever they want and then inject insulin to cover it.
However, the problem with this advice is that usually blood sugars will swing high and low all the time, which is very bad for long term health.
"Can blindness, neuological complications, or anything like that occur from her doing this in the future? Thanks in advance! "
Short answer to this is, Yes, they can.
They tell people with diabetes that a hba1c of 7,0 is ok. Guess what, it may not be. It is known that risks of complications start rising dramatically as blood sugars / hba1c rise above 5.0.
There is a new approach, which involves eating low carb / paleo or even Keto. With this approach blood sugar control becomes easier and the swings much less. It is also safer.
If you are interested you could take a look at the "Bernstein Diabetes University" site on u-tube. He is one of the key proponents for eating low carb and having tight control. Once you understand what is being said, maybe you can try to share with her.
Being that she is 16, you might need to take a subtle approach.... for her to accept what you are saying.
There is also a typeonegrit facebook page, comprised of people with type 1 diabetes, all managing and being healthy eating lower carb. Ask her if she is interested.