@ThemBo You know it's a bad idea to leave makeup on overnight, right? It can really mess with your skin in a number of unpleasant ways; I can't think of too many women who'd be all that upset about someone they trusted removing their makeup in a situation like this.
More to the point, the prince doesn't really trust
her and probably found it more than a little suspicious that she's wearing enough makeup to hide the natural sheen of her skin. Look at it from his perspective: she's hyper competent, comes from a place he literally describes as hell on earth, disguises herself (to a degree) to make her appear more ordinary, and has easy access to royalty. That's like the perfect orgy for being an assassin.