I'm no lawyer but I'm pretty sure that PCJ doesn't apply in this instance. If she didn't give evidence in sworn testimony, then she's not committed perjury and, unless she was also a suspect, she wouldn't have been given a police caution so anything said at the police station couldn't be used...
They're obliged to. Everyone has the right to a fair trial: you're on a very slippery slope if you just say that someone should be denied a trial because he looks funny.