I would suggest that the victim applies for a judicial review of the Crown Prosecution Service's decision, perhaps based on them failing to take account of relevant considerations and/or procedural impropriety (of the police) possibly. I am only doing my law conversion course (training to be a solicitor), so please do not rely on my advice here, but there would seem to be a case for action. The best thing to do would be to get professional advice from a solicitor/Citizens Advice Bureau.