Reg,

Ah, sadly, it sounds like your dear friend is in the same place dear people I have had in my life have been in.

Growing up, I had this lovely uncle who did so much for me over the years, and his alcohol consumption was at the same level you described.  His rehab stints were also the same.  He would agree, stay a couple of days, then leave.  This went on for decades and there was no way to ever reach him. 

Soon, the alcohol consumption was everything to him. 

Eventually, my Mom found him dead at 46.  No one could reach him and sadly, some people are just that way. 

My own father was also an alcoholic, but not to this extent, but also bad.  He also would not give it up. 

My point is - I don't think there is really anything anyone can ever do to persuade a person from a lifetime of things like this until that person decides to go all in and you mentioned you already know that. 

Sadly, I think the physical body learns to "need" that addictive thing to just function and the brain knows it, whether the conscious does not. 

So, in other words, to me, about the only thing one can do is just stand aside and be ready to help pick up the pieces and do whatever one can without harming themselves by being pulled into the vortex of uncontrollable behavior of others.  

I used to hate dealing with people that were huge assholes when drunk or high but really decent people when sober.  Part of that was because too often, the only times others knew of these people when they were asshole due to the substance they were abusing.