Think I should tell an ex friend that her boyfriend is being unfaithful?
Why is it necessary to ruin her life?
I want to because it will make her sad.
I understand your concern, but your reasons for doing this is scummy. She is not your friend anymore, so you don't need to keep tabs on her life and she doesn't want that either. You need to butt out or she'll hate you more for ruining her relationship.
I want to hurt her.
You need to move on with your life and just let her be. It's bad what happened to her but you aren't making it better trying to add salt in the wound.