I recently went through this with one of my best friends. I know how you feel; it is hard to watch someone you care for make mistakes, but I gained a great testimony from it.
I prayed to Heavenly Father to help me know how to help her. The next day when I saw her at school, I realized that by being her friend and being an example to her, I was helping her. I was there when she needed me, and after she stopped dating, she told me that she was grateful for my example and how it had helped her see what mistakes she was making in her own life. Be someone she can turn to and talk to about anything. Don't judge her—love her like Christ loves you.
Myrisa G., 17, Montana, USA