Why can't I run other peoples lives?

I know what's best for everyone but myself, so why can't I just be in charge of someone else's life for a while.
I have this friend, she has been my friend for longer than anyone else ever (other than my brother). She is ruining her eternity. I can't say she is ruining her life (although it is certainly a possibility) because plenty of people in this world are doing exactly what she is and they think they are just fine. I know better, and she knows better. She just doesn't care right now.
I taught the Gospel of Jesus Christ side by side with this girl. We studied and prayed and struggled together to teach people we cared about why they shouldn't do the things that the world sees as ok. And now she has not just slipped into these things gradually or unknowingly she has jumped with both feet. I would never in a million years be judge her for I myself have struggled and slipped on occasion. But I hurt for her like a parent for a wayward child. If you read my post about teenagers you know how I feel about this. I have too many Sisters that have gone down this path I'm not sure I can handle one more.
I suppose since I can't run other people's lives I will just add one more person to my prayers.