You'll want to help her see a lawyer to initiate termination of parental rights proceedings.
His criminal record, her testimony about the assults and beatings, and the history of a conviction for a sexual offence will all be admissible. Courts impose a VERY high standard before terminating a parents rights, but they will do this.
Your friend should be aware that if the court terminates the guy's parental rights, in most states that also removes any obligation on his part to child support. Most lawyers who are trying to get a dangerous man/woman away from the kids use that to their advantage: I.e., the guy gets served with a petition for support and payment of 1/2 of the prenatal and delivery expenses, followed by a visit from the lawyer saying: "Here, sign this voluntary surrender of your parental rights and we'll drop the whole child support thing...."
Before you help her with this, be aware: YOu're only hearing a part of the story and as much of a shit as the guy clearly is, termination of his rights to his own flesh and blood is a big deal and should not be approached lightly. Also, if he's got money (a trust fund?) or some sort of good income, letting him of the hook for child support will not be the smartest thing to do.