Funny it may seem, but Saeril was expecting this sudden outburst to come from the leader of the company. When Thorin began to speak in Khuzdul, she understood his every word, and this wasn't pleasing for the king nor her. Yavanna was the only one to be exasperated for Thorin's behavior, and Saeril looked to the ground, staying out of the conversation before she could manage to look up again, only to witness the illusion upon the King's face. 

The change didn't frighten Saeril and, when she noticed the princess step back a little, the elder looked back to her with sympathy, before reaching out a hand, offering her the comfort, as well as the courage to step up again. In fact, the illusion only struck Saeril with pain for the ElvenKing.