My brother dated someone for three years, and everyone thought they would marry. I never liked her because I watched him become quieter around her. He stopped seeing friends, changed hobbies, and constantly apologized for things that weren't wrong.
When they broke up, he was devastated. I hugged him while secretly feeling relieved. I never celebrated or said "I told you so." He eventually recovered and is much happier now. But I still feel guilty that during one of the worst weeks of his life, part of me was grateful.