I know someone who's dad is raping he what do I do?

You tell the local authorities or your mom and dad so they can get in touch with the police.

I'm not sure if this in "child/teen health" because you are a child or the person being raped is a child - if you are a child your best course of action is a counsellor at school. If you make the Police Report and it's unfounded (will your friend say it is happening or deny it?) you will cause yourself a gret deal of trouble. Go to someone at school and let them file the report.

Your parents can, likewise, report it - but it will be third hand information, sometimes not taken terribly seriously.

If, of course, you are an adult, then go to the Police.