Why would he care if she's an oni? After all, he's an oni himself, and not just any random oni, but the titled Red Oni. In fact that also qualifies him as a potential marriage partner for King Enma's daughter.
She had to go back to her father because her identity was exposed but the one that caused all that fiasco was the fiancee... Damn right the engagement should be cancelled, dumb giant can't even respect the wishes of his future wife.