@Annabel91 - prawn may not have seen your post because the threads I'm on link is dodgy at the moment, so I'll have a go with the understanding that she will probably return at some point with a much better her own answer.
You might have missed it up thread, but I'm also a recovering alcoholic with 25 years of sobriety.
If he were to mention one day that he’s not drinking any more, do you think it would be most helpful for me to say something like “Wow, that’s brilliant!”, or to act as if it’s no big deal?
It depends on where he is in his recovery. If he's not drinking because he's really in recovery then feel free to say something positive. A bit of encouragement is generally welcome. If he's not drinking because he just got arrested for punching a cop while drinking, it's probably best to just nod and smile and say nothing.
I tend to operate under a "need to know" rule. My doctor and my DH need to know I'm a recovering alcoholic. The rest of the world just needs to know that I don't drink. After 25 years, not drinking isn't a daily struggle, it's normal.
You have mentioned that alcoholism is an illness. I’m wondering how exactly the friends and family of an alcoholic should think of it
Like any disease, it can be active or in remission. In the case of any addiction, the long term treatment that creates remission is abstinence. Alcoholics do have control over it in that they can seek treatment (medical or peer support or both) and devote energy and determination to staying abstinent. There really is no issue of blame, any more than any other disease. The only distinction that I can think of is that most medical treatment tends to be fairly passive - take these pills, etc. Because alcoholism is so behaviorally intertwined it requires a change of habits and attitudes, which is much harder than taking a pill everyday.
I generally don't care to talk about it with people at all, except with others in recovery or in situations like MN, where my discussing my own experiences might be helpful to others. If he is sober, he knows what he's doing and living life without planning his day around alcohol and waking up hungover is it's own reward. If he's struggling and brings it up, try to be supportive and encourage him to get help. Very, very few people who are actively addicted can stop on their own. There's an old saying in AA which goes "You alone must do it but you cannot do it alone".
Reading this over, it seems terribly unclear, so please ask if there's anything I can expand upon. And thank you for being so considerate of your friend.