Should I tell her...

Mary • My beliefs and opinions are constantly evolving
I saw one of my friends in the mall with her boyfriend (they've been together for 3 years), but then I remembered how he cheated on her for who knows how long with a girl who went to a different school (one of my friends caught him). 
I remember when my friends told me the secret freshmen year, she told me not to tell the girl which I didn't understand why since what he was doing was wrong. 
I am not sure if he cheats anymore, but I feel bad and want to tell the girl that he used to cheat on her. I know it's none of my business, but I feel like she should know... Should I tell her?

Vote below to see results!