I have this horrible horrible feeling that the duke is banging this maid. You can't parade or publicly shame the missus in dirty clothes without that reflecting poorly on the husband. While the duchess has already done some of the work herself with her anxiety, to take it a step further like this is dangerous. So to do so brazenly and confidently feels to me like she has backing from her employer to do so, the kind that comes either from sadism or intimate relations.
That said, that slap is SATISFYING and I want more.