Can I tell an ex's fiance that he cheated?
Do you think she is unaware of his cheating?
Yes she might have, but I don't like her anyway. She's cancerous to me.
You should not get involved in other people's relationship. Telling your ex's fiance that he cheated on her will make your appear jealous because your intentions will be clear.
Why not? It will make me feel better.
You would be ruining a relationship just so that you would feel a little bit better? A lot of people would say that you're wrong and think less of you for doing this.