When the Thorpes can't convince Catherine to cancel plans with the Tilneys for a scheme of theirs instead, John Thorpe goes and lies to the Tilneys and does it for her. Catherine promptly turns around and runs after the Tilneys to set the record straight, refusing to let tricks or lies cheat her out of what she really wants. If it was just for what she wanted, she might have done it, but she did it because of her morals. She refuses to break her morals.
Catherine instantly seeing through Isabella's letter claiming she's still in love with James, and declaring their friendship over.