Ethical Dilemma #2 - To Tell or Not To Tell

If you knew a guy* was casually cheating on his girlfriend of 5 years, would you tell the girlfriend?

Fortunately, I wasn't faced with this as I only found out this guy cheated after I met and spoke with the girl.

If I were her, I'd want to know so I could move on and find someone better. However, I don't think I'd be able to tell her as I barely know both of them.

* works with my husband