The fact that Donna had cheated on John did not affect the reality that she felt remorseful for what she had done. He couldn’t understand why Donna felt the need to cheat on him after everything they had gone through. At the time, no one was honest.
To make matters worse, meeting Donna had just exacerbated his feelings of betrayal and loss. Due to Donna’s condition, she had to depart with a sad heart. Donna’s best movements were yet ahead of her, despite John’s belief.