Fuminori could tell Toothless--the Mirror or the Real--what a Real's blood tastes like. He's sitting outside, eating what to anyone other than him will appear to be lumps of bloody flesh. With everyone working on the riddle, there aren't a ton of people wandering around the grounds, meaning he doesn't have to deal with the worry of people seeing him in the middle of a meal.

Outside isn't so bad, because the sky is just pitch black, and as long as he chooses a relatively bare place to sit, the scenery isn't too bad either. It's boring, but at least it doesn't make him too nauseous to eat.