2) On no account invite the formerly inebriated one to a pub. They will certainly succumb to alcohol. It is, after all, the lack of availability of alcohol, rather than a considered decision, that has led to them not drinking.
3) Don’t mention alcohol, or its effects, in the hearing of the now sober person. They cannot identify with anyone who has enjoyed themselves drinking, nor with anyone who has a hangover. It’s not as though either of those things have ever happened to them.
4) In the event of forgetting 3, it’s best to stop talking altogether, look embarrassed and change the subject. The ex-drinker will definitely not notice.
5) Tell the no longer permanently hungover person how much you drink. They will be able to give you an instant diagnosis of whether or not you are an alcoholic.
I honestly couldn't think of a picture to put in there, so thought I'd follow Sam's example and put in a cute cat pic to try and boost the ratings. It's cheap I know.