My first thought would be to do it exactly as Muppet suggests. But you know your friend best, so you'll have to figure out how to approach it with her. I would certainly tell an adult as well, especially if she doesn't listen to you.
Do you two have a mentor or instructor that could explain the problem to her as well, so you're not just like "Some people on the Internet told me it's a bad idea?"
Then again, I guess you can just outline the reasons why it's a bad idea, and hopefully she'll understand that it is a bad idea.