She was damaged with the loss of her daughter. That is not hard to understand. It would be, “I forgive you for taking my daughter away from me”.
That is a huge damage. The survivors are the ones who end up suffering. For the dead, they have no consciousness to suffer. They are just on to the next phase where we are all heading eventually.
Check this out:
http://www.catholiceducation.org/en/religion-and-philosophy/philosophy/forgiveness-is-a-kind-of-wild-justice.html
It is not her place to forgive the violence done to her daughter. To the letter of the law, there is no crime done to the mother. Of course, that does not mean that she has not been made to suffer. By extent of your argument, as the daughter no longer suffers, there is no crime, only one against the mother.
Regardless, no matter who forgives whom, there is still a penalty to be exacted and a price to be paid by the law. To set this man at liberty because the mom says no foul is to make a mockery of the law.