Do You Tell or Keep It To Yourself

I have a friend that is dating a guy that I know isn't the guy she thinks he is.  He has a history of not so nice things, he's known in circles as the 'hot head' and 'player' but as of right now, she has no clue.

As a guy, I would want to know but I'm really afraid it won't be received so well by her.

I asked around our building and the consensus was about the same.  Girls said that unless the person was a good friend, it's not their place.  "Once you say something, what happens after is your responsibility".  Another said, "I would say something but have backup people that didn't know the person you are telling to verify."

Guys seemed to have the same answer, "it's none of my business"

I've been in this situation and had friends tell me after the fact that they knew she wasn't good.  When I asked why they didn't tell me, they said they didn't think it was their place.

I guess I'm asking, should I say something or not?