(2) I don’t want the best friend/romantic interest character to come as preachy and I don’t want the rebel character to be romanticized. What should I do?
Make it sound genuine.
If Character A loves Character B, A will not start making speeches about what B is doing. She’ll be genuine in her concerns and it may come up slowly. However, how A approaches this also depends on her personality. She could try to bring up her concerns casually or, if angry, she could sit B down and try to put some sense into them.
To avoid romanticizing B, make the portrayal accurate. Look into the effects of the drugs she’s using and think about how this behavior would change her psychologically. I would suggest watching the film Thirteen as it deals with similar topics and it’s handled well.