She might not be prosecuted for the same reason that the La Cross students should not have been prosecuted.
In order to be prosecuted the prosecutor must not only believe that the defendant is guilty of the crime, but that he or she has sufficient evidence to prove that fact beyond a reasonable doubt.
In this case the attorney general suggested that in his opinion the accuser may actually believe some of the stories she told (i.e., she is delusional) False memory syndrome. Etc.
If the prosecutor believes that he cannot convince a jury that the defendant had formed a criminal intent, then he cannot prosecute the defendant.
Nifong knew that this woman was lying, yet he went ahead and prosecuted the students. That is why Nifong will spend a few years in jail and the accuser is probably going to walk.
There are two researchers who have conducted the most important work on false memory: Beth Loftus and Stephen Ceci. None of their work supports the notion of multiple, mutually exclusive false memories.
It is up to the jury to decide if the false accuser is a liar or is insane.