It's perfectly natural, at least in my world, to see someone who is engaged in totally reckless behavior, and want them to stop it, not only for themselves, but the life that they save, maybe yours out on the highway some night.
You can't help someone who doesn't want to be helped or even realize that they have a problem.
Al-Anon/Ala-Teen is an organization, completely anonymous, that is there to help people who have family and friends who are alcoholics, or might be alcoholics (They were a great resource to me as a teenager) The organization is anonymous, you might want to consider going to a meeting. Just to check it out, see what they have to offer.
Does she have other friends that you are friendly with, if she won't listen to you, will she listen to them? Does anyone else see the problem that she has developing?
Write her a letter, anonymously, pointing out the behavior that you are seeing, and what the consequences might be.