Can I not tell my colleague his fiancee cheated on him with my ex?
Why do you want to keep the cheating a secret?
I have a decent relationship with my colleague and don't want to make it awkward.
It is possible that telling your colleague his fiancee cheated would make things awkward at work. If you're not close friends with your colleague this might be one of those secrets that are better to just keep a secret. You really shouldn't be discussing his sexual relationships at work anyway.
Nah, my ex was wrong for cheating on him. Since he was with my ex for years and never cheated I think it's fair.
It is a bad idea to bring this up because it involves coworkers. You should only discuss important professional issues with them generally.