I had a brother-in-law I loved deeply, but he was an alcoholic. He would show up at our house drunk and crying at 2am and I would sit with him for hours talking.
I finally told him he had to get some help with his alcoholism and I felt I could not help him enough. He refused to let me get him some help and so I had to distance myself from him for my own sanity and health. A couple of years later, he went to the doctor and got off the alcohol and graduated from college with a degree.
You may have to stop seeing your friend if he will not get the help he needs. You can try to do an intercession for him, but if that fails, ultimately he will have to decide on his own if he really wants to get well.
I think you should get a way to take them home if they are too drunk. You should not just leave them alone since you care about their security. However, if they do this every other time, talk to them about this or avoid their company.
Short and quick is the best way. Just tell your friend: You've got a problem and unless you do something about it, there is no way I can be around you. I say do it quickly because their first response is I am sick, and how can you abandon me now. Don't fall for it.
I liked HeresKitty's answer. I have been the drunk friend. All the love, tough love, tears will not help unless the person acknowledges the problem and wants to address it. As my ex told me, "I'm not trying to hurt you by leaving, I have to save myself."