It seems to be a bit of a shock for Sophie to discover not only that Mrs. Fairfax knows who Howl is, but that Mrs. Fairfax also doesn’t fear Howl or think he's a bad person. Indeed, she seems to think highly of him—and think he’s trustworthy, if she’s encouraging Lettie to accept his romantic advances. Sophie brushes over this a bit and instead focuses on making sure that Lettie is getting what she wants out of this relationship (which, as Sophie understands, is this other man who’s under a curse). This highlights Sophie’s loyalty to and love for her sister.