Well you didn't make it clear in your thread starter that you are in the US; one of the most powerful deterrents we have here is that prostitution is legal, and potential wrongdoers know that there is no reason for us not to report if they threaten, abuse or harm us. Brothels (which are illegal) get robbed more often, because the offenders think they won't dare report incidents, and recent legislation like the Proceeds Of Crime Act haven't helped.
If working as a prostitute could compromise your friend's future career, she must have known that when she started and decided to accept the risks. I still cannot think of any reason for not reporting a dangerous and violent offender - drop all the advertising, see that he goes on all the relevant warnings boards/sites and make sure the authorities are aware of the need for her identity and whereabouts to be protected. The guys who have been picking bits of women up off the beach on Long Island would probably agree too.
I also agree with Rooby - whilst some people may not mind recounting personal experiences your repeated request does seem to show an unnecessarily prurient interest and since any responses will not be relevant to your friends circumstances, no amount of tinpot cod-psychology from you or anyone here is likely to help.