If he really wants her to see hell, he needs only send her back to the spawn with his words still fresh in her mind. Some of the others are bound to start asking questions if he never shows up again, while she's stuck questioning her actions. Even if she convinces herself that he was an impostor, others will still hold their doubts and blame her for "killing" a classmate, ultimately causing the group to fall apart and delaying their growth.
Instead, he's going to have fun tormenting her in the moment and come up with some excuse how he escaped and made it back to the city, isn't he. He already missed his chance to explain the situation and bring her around to his side in delaying their escape.