Part of understanding forgiveness is to understand what it is not:
Forgiving her abusive husband does not excuse or condone his cruelty.
Forgiving does not mean forgetting his brutality; you cannot unremember or erase a memory that is so traumatic.
Forgiving does not mean that justice is being denied, because mercy cannot rob justice.
Forgiving does not erase the injury he has caused, but it can begin to heal the wounds and ease the pain.
Forgiving does not mean trusting him again and giving him yet another chance to abuse her and the children. While to forgive is a commandment, trust has to be earned and evidenced by good behavior over time, which he clearly has not demonstrated.
Forgiving does not mean forgiveness of his sins. Only the Lord can do that, based upon sincere repentance.