Lose to the boss in the first dungeon for that scene. You can also check the scene reviewer for many of these, it is just for the scenes with artwork, and all the art for each dungeon unlocks after you clear it.
Yeah, I discovered that a bit later by purposefully losing to her. It's a pretty nice scene, wish there were a similar one with randomly encountered orcs or when napping in one of the rooms, only more consently and intimately. Like a possible variation from being caught napping by the futa orc :)
