I’m really sorry to hear about your family member. Unfortunately, it’s impossible to give you an answer to what I think you’re asking. I suspect it has a lot to do with his underlying health issues and his mental state.
The body is surprisingly resilient though and the liver especially is very regenerative, to a point. To give you an idea, before I quit drinking, I was drinking well over 100 units a week. I’ve just done the maths, 30 units a day x 7, that’s 210 units a week.
I stopped with no issues. My liver function tests were as normal as could be 4 weeks later. And I don’t have any (as of yet) health issues that I didn’t have before. Obviously, lots of things can creep up in middle and later life (I quit in my 40s), but you can stop as a very heavy drinker and live a largely healthy and normal life. I know lots of people who’ve done including many who are older than me.
So all hope is not lost. There is a way out of this for him. His other long term conditions can be managed and will be much easier to do so when he isn’t drunk or hungover. But obviously he has to want to change his life. That said, there are many people who can drink 100+ units a week for decades, with obvious health consequences, but alcoholism is often a slow degenerative condition. It only gets worse unless the person makes it better, but it can get worse and worse for a very long time unfortunately.