This, they say, precludes his smoking in that room, meaning that he will only be allowed to smoke in his bedroom, even when the pub is closed.

Assuming this isn't one of those "friends" who crop up in innumerable urban legends, I'd advise Don's friend to have a good laugh and completely ignore the buffoon who said this. I am pretty certain that any attempt at a prosecution in such a case would be treated as an abuse of the court system.

And I have no doubt that a group of friends meeting together in non-public premises, without any paid staff being present, would have no legal problems whatsoever if they wish to smoke. Well, smoke tobacco anyway.

It would of course have been much better if in the first place the push for limiting smoking in all places where it might inconvenience other people had come from smokers themselves.

I have never been aware of any occasions where smokers have demanded that separate smoking facilities are provided, so that they can smoke in comfort without interfering with the comfort of people who were not smoking. At least, there were no such demands until the objections of the non-smoking majority had produced an irresistible momentum for a ban.