Wiktionary
done thing
n. (qualifier: with definite article) Something done habitually or according to social norms, expectations etc.
Usage examples of "done thing".
It is not the done thing to apply to a Sorcerer for help in locating people, unless the Sorcerer actually hires himself out for that sort of thing, which very few do.
It is not a done thing, something that can be performed for convenience.
Of course he'd never let on, that wouldn't be the done thing for him, but it's there.