Despite her bored state, Alessa had also been alert. So when Blythe escaped from the chair, and proceeded to throw it at Kain, Alessa wasn't prepared. The chair hit Kain before Alessa could do anything, and she quietly cursed to herself. She realized that if she waited for Kain it might be too late.
Even if Blythe was dressed like some sort of 19th century Victorian woman, Alessa feared that she would be able to move pretty quickly. Without waiting, Alessa quickly ran out of the room, and gave chase to Blythe.