Lock them both up and throw away the key.
Part of me wonders, though — What are the odds that the girl’s father (the type of man who had a daughter with a prostitute, mind you) just happened to acquire the phone just before the crime was about to go down? It’s paranoid, of course, but that sounds more like a cover story for information acquired through telecommunication surveillance than something that actually happened.
You assume he knew she was a prostitute. He may have thought she was just running around on him only to find out this.