Heart breaking, beautiful. You did all you could. I have been the friend coaxed into wildness, flirting with disaster, and the truth is that as much as the friendship meant to you, and them, it can't last forever with a person like this on a downward spiral. No one can be saved who doesn't want to come out of the dark. It's horrible that life wrecks people we love like this, and we witness it, powerless. But to stay with her on that ride would have ruined you. Try not to regret things that were not in your power. I don't mean to sound harsh, I have just had similar if not quite as dramatic experiences with female friends. People who walk away remember everything, you remembering her here is a true tribute. You didn't fail her, you did your best at the time and you saved yourself when you sensed real danger, there is absolutely no shame in that.