I get the feeling throughout these chapters that FW is closely watching Anne. Although he may no longer be attracted to her as a lover, he still gives her consideration.

..."and Captain Wentworth, without saying a word, turned to her, and quietly obliged her to be assisted into the carriage."

I think he is looking for clues to her personality and perhaps wants to find some reminants of the things that once attracted him. We must assume from his other comments that he also feels sorry for her (never dances...would not have recognized her etc.).