Sarah was stunned regarding this woman as a monster. She seemed so complex. From the beginning of their relationship, she had suspected that Charles didn't love her and maybe loved another woman, but she had no evidence. Equally, Charles's extravagant displays of affection had distracted her from her suspicions. However, today she was told directly by this woman that Charles didn't love her.
This woman was dating Charles. Charles was cheating on her. What's more, it was consistent with Sarah's first instinct.