What do you do if you know about a child who is being smacked around by her father and she's 17?

I know of an situation where a family friend was trying to protect her daughter's 17 year-old friend from the friend's father. The friend didn't want to go home with her dad because she was afraid.

The police were called. When they arrived and heard the story they said the girl had to go with her father because she was under age.

The police said the child has to have proof on her body that her dad has been smacking her around.

Come on...who protects the children?