Sorry for your pain and your friend. It's SO SO SO hard to watch someone drown and you can't do a thing
I have a friend like this who is a coke addict. I've had to detach from her over and over and she will stop using, then goes back to it.
And here's a real kicker, I've not spoke to her in 3 months and she showed up at my house the other day to tell me her Mom just won over 2 million in the lottery and she was given a 100 grand. I suspect this will be her death sentence.
Just detach and wish them the best, all u can do