In addition to the mediation, it will take years of sincerely showing they have changed in order for the child to forgive this level of abuse and neglect. In fact, the child might never forgive the parent, but the parent can't do anything but try in the hope that they will eventually be forgiven.Cannot be solved without a mediator
A third party has to mediate, tell the child that the parent does not really mean it, and facilitate a meeting between the parent and the child.
I hope Sayu doesn't forgive her mother during this meeting, even if her mother somehow sees the light and admits the error of her ways. Otherwise, it would make everything Sayu experienced feel extremely shallow. It would also send the extremely wrong message that the abused are supposed to forgive with just a simple apology.