I don't know the girl let's call her Mary. Mary's. Dating one of my friends' friends. We do come together sometimes and chill sometimes go to bars. Mary is very affectionate publicly with her SO, they're constantly on top of each other but because she travels a lot for work and is often out of state she has the habit of bringing over other women and I only know this because he admitted to my friend. And I don't feel like it's my place to break the promise I made to my friend and tell Mary what was happing. I don't know if it's still doing it but they have started making plans to get married and even to have kids.  should I just keep my mouth shut. am I an a****** for just trying to not get in the way of their happily ever after?  

TL;DR guy shooting on this girl I know but I don't feel like it's my place to tell her what's happening