"Do you sing that song a lot? Is that where you usually pitch it? I was just wondering if you might want to try it a touch lower down. It can be hard to nail those high notes every time".
spot on, that is exactly how constructive criticism should be given.      
correcting diction or even ACCENT OR IF YOU WANT DIALECT, is part of constructive criticism of singing, again the way to approach this would be for example. I really enjoyed your version of nicotine girl, but did you sing "down the wymondham way"
i might be wrong ,i am no expert but i think it is generally pronounced windham.
again this sort of thing is best done on a one to one basis, not in front of a whole club.
incidentally an example of MacColls arrogance or rudeness, the lisa turner incident, telling her or maybe even interrupting her when she was on stage, to inform her that the club had a policy that performers had to of sing songs from their own country, that sort of thing should be done afterwards and in private.