<human>: Give me the gist of the text in a nutshell

My boyfriend's best friend has been dating this girl for over a year. For the first 6 months or so, they were obsessed with each other and everything seemed fine. We have gone on numerous double dates and hung out together (the 4 of us) several times, so I've gotten to know her pretty well. She is the sweetest girl and incredibly gorgeous; I don't know why she stays with this ass clown. Somewhere along the way, they started having a lot of drama in their relationship and broke up for a little while. When he started hooking up with other girls, obviously, no one cared at first, until his (ex?) gf started posting things on FB about "her man" and having date nights with the guy who had supposedly broken up with her. Come to find out, he is still leading her on and letting her think they are in a relationship while he tells everyone else that he's single and is hooking up with a different girl every weekend (I have witnessed this myself on more than one occasion, and heard several stories from my bf and his friends). Even my bf and some of their mutual guy friends are upset at this guy for what he is doing to this sweet girl, but no one has had the balls to really do anything about it.

My question to all of you: Do I have the right to say anything either to him OR to her about this situation? Can I tell her what a lying douche her "bf" is and how he is hooking up with/pursuing a different girl every weekend? Or should I just stay completely out of it? Any of you ever experience a situation similar to this?
<bot>: My boyfriend's best friend is cheating on the sweetest girl, and everyone knows it but her. Should I inform her of his cheating ways?