Rarity doesn't say much of anything. The imminent danger is gone, yes, but that doesn't mean this is anywhere near over for Sans.

At a loss for anything better to do, she takes a seat next to Sans, tucking her legs under her. After a moment, she magics a hand towel from the kitchen to her hand and starts patting Sans dry, gently as she can manage, humming quietly all the while. It's a song she only vaguely remembers, but she knows it is a lullaby; perhaps it will help soothe him.

She'll let Papyrus do the talking for now; he likely knows Sans better than she does. She has to believe he'll know what to say.