You expressed your concern, and that's all you can do. All you can really exercise control over is your own actions, and in this case, your realm of control included expressing your concern, not actually preventing her from doing coke. She's the only one who can decide for herself to do or not to do coke.

It's sometimes a hard lesson to learn that just because you say something doesn't mean people have to actually listen, change their own behavior, or do what you recommend. But the sooner you learn that, the less frustrating a life you'll have in the future.