Nothing you did. Nothing you could have done to prevent it. This just illustrates how terrible such drugs are. They take perfectly normal people and destroy them.
Nothing he could have done either, short of not trying it the first time. He doesn't want to be the person he is now, despite appearances. He just doesn't see a way out. And at this point, he may not care to. Just try to remember that's a product of the drug. Not who your friend is on the inside. Although sadly that may change in short order. I hope it doesn't.
If I were you, I'd stay available. Check in on him from time to time, but don't hang out. Let him know you care for him, let him know you love him, but don't abide the lifestyle. Don't subject yourself to the pain of watching him deteriorate. Don't subject yourself to the danger of involving yourself with the characters he's chose to associate with. It will only hurt you, and it won't help him.
One day, he might reach rock bottom & reach out for help. It will be important for him to have a friend who hasn't forgotten him when that day comes.
Have you considered an intervention? I say that with the disclaimer that I have zero experience in such matters. I am, in fact, curious what others here who do might have to say on the matter. But considering the potential consequences of crack addiction, it might be worth looking into. There's nothing you could do to him that would be worse than what he's doing to himself.