There's unfortunately a few things at play here. Its cleat that whom ever this is, hypothetically of course, is totally OK with it. That means while its an addiction, its sort of....not. I mean, they can't necessarily stop, but they don't want to. Its like me and corollas. I'd call it an addiction, but I don't want to stop hoarding them. its fun, and its not LBCs. In essence they're OK with their personal demons. Most humans, (even the imaginary ones) are smart enough to know things like, crack is wack, yo. Whether or not they want help is another thing entirely. Case in point, my father. Total raging alcoholic, drugs, you name it. Probably well aware of it. Most functional abuser on the face of the earth.
That's not to say its not without its caveats though. He's dead because of his vices. Turns out in face vs. Car, car wins. Specially when face has been launched from motorbike. Many people tried to save him, as it were. His close woman friend even left him over his addiction. He was dead less than a year later. However, I don't believe that anything she or anything else anyone could have done would have changed the outcome. Why?
No matter what your say, threats, cajoling, handjobs, etc- no one is truly going to get help and attempt change unless they want to. Datsuns brother is an excellent example. He likes being loaded. He knows its going to kill him. He's accepted the consequences of his actions, and still chooses to live that life. Now, most aren't as...jovial about it. Many have enough personal demons that the thought of sobriety far outweighs the possibility of death. Never been an addict, but I can understand. Escapism is a powerful drug. And gin is cheap.
Now, as far as what you can say. You've first got to make your own bones with this. I'm of the mind that you can still love a person while not being able to stand being around them. Just look at my wife some days. However, totally writing them off? I don't think that's realistic. So, what I would do is- sit down and truly weigh out the thought of distancing this person out vs. Seeing them in what amounts to self destruct mode. Datsuns case is an interesting example. Total raging alcoholic, super fun to be with. My dad was the same way- but he also his his problem very well. Can you live with the knowledge that they're doing this and that they're OK with it? Its a tough sell, but love is a powerful thing. If you can't, well- peace out girl scout. If you can-
Something like "look. I know you like to get all helped up on goofballs.it hurts me to watch this. (Maybe explain why) but- I care. So, I am still here. If you ever decide you wanna not be hepped up on goofballs, let me know, I will help you. In the meantime, businesses as usual" or along those lines. Let them know you have an difficult time with it, but you still care enough to be there. You.never know, that could be The tipping point. Dealing with negative life issues via substance abuse is common, especially abandonment. If you stick- it might give the person a bit of hope.