At first I thought they were a boy. Then when they were in the hallway, I thought that was a different person he was talking with that was a girl. I don't know why I made that mistake.
Of all the hints people have talked about. I can't believe that no one's mentioned the part when he's worried if the person can eat them. She literally answers by saying she likes them a lot.
Edit: Wait, I just reread that. I think that he realized the truth then.