At what point do you let go?

I am pretty sure that most people know someone that abuses some kind of substance.
Be it alcohol, weed or any harder kind of drug.
A mate of mine is addicted to Cocaine.
He is at a point where he has lost his job, lost all his friends and even if he has not realised it yet, is about to lose his paternal rights to his daughter.
So I was doing some research on the subject, as there needs to be a time where you say enough is enough!
Functional drug abusers are one thing, but to sit back and watch a friend hit rock bottom is worse. Hell is trying to help them and being slapped in the face around every corner.
Extracted from coca leaves, cocaine was originally developed as a painkiller. It is most often sniffed, with the powder absorbed into the bloodstream through the nasal tissues. It can also be ingested or rubbed into the gums.
To more rapidly absorb the drug into the body, abusers inject it, but this substantially increases the risk of overdose. Inhaling it as smoke or vapor speeds absorption with less health risk than injection.
Popular street names include Aunt Nora, Bernice, binge, blow, Charlie, dust, mojo, nose candy, paradise, toot and white.
“Addiction should be understood as a chronic
recurring illness that requires treatment.”

Having watched endless episodes of Celebrity Rehab with Dr Drew, I did manage to understand some of the issues surrounding someone addicted to a drug of this nature.
The respect for themselves hits rock bottom, they lie, they cheat and they lie a little more.
All to get to the high that they think will get them out of the hole they are constantly digging for themselves.
So where to from here? Is rehab the only option? Does rehab work? How do you get to someone that is so far down the road that they need more than just an intervention, because if left alone after a 12 step programme, they will head back to the same dodgy street corner to score?
There are so many prescription pills on the market that are there to assist people to get off the harder stuff. But this is a never ending saga of going around in circles. I knew a guy that gave up booze only to chain smoke himself to death. Where do you draw the line?
What are the deal breakers for friends of addicts?
When do you let go?