She doesn't say a word. Her reaction could very well be "That bitch!!"But during that whole scene in TOS Chapel looks totally surprised. She doesn't seem to even know Tpring existed. All during TOS chapel or Spock never give any indication they were previous lovers. They act at times like they hardly know each other.

Their relationship cools in SNW season 2 with Chapel being very distant on purpose. We'll soon see if the cliffhanger changes that.