Nothing and nobody is all good or all bad. Even the most evil people are capable of doing heroic acts of bravery.
Likewise even the most angelic of people committed to acts of kindness and charity are capable of despicable actions.
Where we like to label and brand people with the scarlet letter of their past behaviors...no one is beyond redemption.
God doesn't care who you were; He cares about who you are.
That being said...there are paths for recovery from violence. Violence is addictive like any other drug. For some people it's like heroine or Crack cocaine.
Just because the OP tells the woman that her current boyfriend was abusive to his last girlfriend doesn't mean that she will react in a positive fashion. There's a reason why abused women stay with their boyfriends/husbands....and it isn't like they enjoy being beaten.
And as
@Magenta has said...tattleing is insering yourself into their relationship.
We all like to believe that we can "make someone good or safe" by just relating pertinent information. And you have no idea how I wish this was true....but it just isn't.
People are who they want to be regardless of who you want them to be.