not the done thing

Kazuo Ishiguroの "Klara and the Sun" を読んでいます。
'I hope Rick does come to this awful party.'
Despite the lateness, I was pleased she'd brought up the interaction meeting, because I was uncertain about many aspect of it.
'Yes, I hope so too,' I said. 'Will the other young people bring their AFs?'
'Uh uh. Not the done thing, But the AF who lived in the house usually attends. Especially if they're new like you. They 'll all want to inspect you.'

"Not the done thing" は前の質問に応えて、「そんな事はしない」と解して意味は通じますが、辞書で確認します。

・Cambridge English Dictionary: mainly UK (US usually not the thing to do): not socially acceptable: Smoking during a meal is not the done thing.
"not the done thing" では上記の辞書しか見当たりませんでしたが、"the done thing" では次の辞書にも説明がありました。

・Collins Dictionary: If you say that something is the done thing, you mean it is the most socially acceptable way to behave.:[British] It was not the done thing. In those days the man was supposed to be the provider.

・Farlex Dictionary of Idioms: A given behavior that is acceptable, appropriate, or correct in a particular social situation. Often used in the negative to convey the opposite.: Calling on a friend when they are sick is simply the done thing.