Go visit her, as it may be the last time you will see her on this earth. There is never anything wrong with visiting someone you had a relationship with if you feel you should, even if she was crazy and rebellious.
Just don't let yourself fall into the "boyfriend" mode as you do it. You are ex's. It is often hard in these situations to mentally stay that way. Caring for a friend is good, to mentally backtrack due to the trauma is bad.
I noted you said if she ever recovered she'd be in the JDC, from which I take you mean the Juvenile Detention Center. Well, hopefully your story here is a lesson. Kids today have no fear of authority (blame that on parents who worry more about being their kids friends instead of being the parent) and thanks to some other influences (video games and YouTube) kids seem to think its okay at every turn to just put anything and everything in their mouths and body.
No matter what anyone else here thinks, I believe that drugs, including maryjane, are inherently bad. Alcohol also, but I've learned in my life how to NOT abuse the stuff. All those things your parents say to avoid, by and large, they are correct.
Kids will be kids, but its time the young'uns here really paid attention. Drugs are a quick and direct way to death. Drinking to excess is a quick way to poverty and accelerant to death. Smoking cigarettes is a highway to hell.
I have these types of conversations every day with my step kids. They don't want to believe me, but every once in a while they get too smart for their own good, get in trouble and have to eat crow. Hopefully they will get the big things and I won't have to be visiting them in the hospital like the OP will be.
Someone actually tried to tell me how cool it was that Hendrix, Joplin, Robert Johnson, Jim Morrison and Kurt Kobain all dying at 27 was cool. I had to remind them it was just pathetic. Same with Winehouse.