I like how the Duke commits some casual Lèse-Majesté and King Mustache can't do anything about it.
What the Duke does there is not lese-majesté. That stuff is for Commoners and possibly lesser nobles.
King Moustache is so weak as a King, the only reason he's still sitting on that throne is the support of the very ducal household he just tried to order to Sloppy Second their daughter to the no-longer-Spare.
That support is currently merely there because the Duke
does not want the hassle of the Throne himself. Any goodwill and politeness already vanished with the betrothal annullment and the way it was handled.
This was merely a Duke "Advising the King" about an erroneous action that King was about to take. As a Noble of the Realm is duty-bound to do..
Possibly in a bit more graphic detail than necessary, but.. you know... doting parents and all that...
And really... There's
plenty of historical examples of exactly this situation..
Never forget that anyone with the rank of Duke is generally of Royal Blood.. Just quite often not in the direct line of succession, but
definitely eligible should one decide to put his foot down when there's a weak/inefficient/insane King...
On top of that, a Duke has military authority
equal to the King, barring the King actually being present on the battlefield, and is expected to maintain his own standing army and levies.
Tradition only gets you so far.... And an oath of Fealty
also puts responsibilities on a King towards his Nobles.
Moustachio has obviously forgotten that teensy but important detail..
And the Duke simply reminded him of his Duties...