If you love this girl enough that you want to help her, then all you can be is a mentor to her. She's not going to listen to your advice (no offense to anyone here, but does anyone at that age listen to advice? I know I didn't at that age), but you can be a shoulder to cry on and take on a "tough love" role for her.
If you're not that close to her and the situation is just nagging you, why not walk away? What will happen will happen.
If you're not that close to her and the situation is just nagging you, why not walk away? What will happen will happen.