You need to follow brothel quest a couple stages up to the point when you'll be able to explore the place. Behind one of the doors, don't remember which but it doesn't matter since you can try/retry all, will be the girl you're looking for. She catches you snooping around but offers to turn a blind eye for a blowjob. You need to refuse so she runs to the owner and you can pick your "punishment" either get get screwed by a bunch of werewolves or the option you want. Pick correct and you'll be transported to strange castle on the map where you will become blond girl blow toy for life. You'll get the scene, achievement gokkun but it's game over.