i don't know. are you taking second hand information from a teenager at face value?
Kearn
What you need to realize is
If he's pulling our legs it doesn't matter what side you pick
If he's lacking info then we aren't far off in terms of how to respond
If she is lieing to him, yes this is all for nothing
If there is real physical abuse going on, this is a case for CPS
Further about the camera thing:
The female would record the videos, store the data and give backup data to OP
OP would not be doing jack stuff
OP would be assisting someone to get out of a possibly dangerous situation.
What you are doing is defending a hypothetical. For all we know OP is Badspot looking for shuts and giggles. I'm glad you're concerned over the legality of a father possibly beating his daughter though, that's really great of you.