Act 3 A Dolls House

In Act three of a dolls house we see that Ms Linde and Krogstad had a relationship. I can't believe she could be so heartless to break up with him and write a letter to him to help him get over her. I didn't like that she changed her mind about helping Nora and convinced Krogstad to not ask for the letter when he had already made up his mind. It's not up to her if the secret had to be revealed or not because its not her life.