Wave Goodbye

It's one thing when your friend starts to get a bit odd and you don't really want to talk with them. It's one thing when your friend starts to get very odd and you don't really want to be with them anymore. It's another thing when your mum tells you she's beginning to not want you to talk with this person anymore.
My friend at first seemed to be acting very immature. Then she began to be very rebellious and immature. And now she is making very immature and rebellious choices.
I don't understand kids, really, especially because I am younger than her yet I see more wrong in what she is doing than she. What is going on isn't acutally wrong, but it isn't wise and not the best choice towards a happy life. I don't really think she is thinking of consequences or anything. Then again not a lot of kids do. It really agravates me when kids, say, drink heavily at night because it's so fun but end up failing a test at school due to their headache the next day. But on a more extreme level.
I really do have big opinions on what I find is appropriate for people at certain ages but I choose not to share them. It just causes too much trouble so all you can do is ask a few 'are you sure you're doing the right thing' type questions, repeatedly remind them they have their whole life ahead of them, pray they'll be alright and wave goodbye as they go off and do what they want, knowing they probably won't come back the same person you knew before.