The Department of Child Protection can not intervene if they don't know that he is living there. This would be the point of you (who have the knowledge) of notifying them. If getting the authorities involved prevents or stops a child from continuing abuse then you would be helping the child in a big way.
If he has access to this child, and the mother is not able or willing to protect the child then notification may give the child a chance. The mother may be able to given resources, or information so she feels able to, and understand her role to protect the child. And if the mother is not able to fulfill her role then someone needs to protect the child.
I don't think there is a need to comment on other parents parenting styles that may differ with no immediate danger to the child, but in this case the child needs to be protected.
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