How many times have cases just been dropped because the witness wouldn't testify?
And likewise, how many times have (adult) witnesses been held in contempt of court for refusing to testify?
The girl's best bet, once she gets a lawyer, is to demand to give a sworn, truthful (assuming it is truthful) affidavit that nothing happened.
This will put the prosecutor in a tough position:
He can either charge the girl with perjury, ruining her value as a witness, put her on the stand where she'll either tell the truth and blow the case or lie and be torn apart by the defense, or not use her testimony at all.
Now, we have no way of knowing exactly what happened. The girl says one thing, the prosecutor believes another. He probably believes something happened because either 1) the girl at one point said there was (she may have been lying at the time), 2) there is some physical evidence (think Monica Lewinsky's blue dress), 3) there is a witness (who may be lying), or 4) there may be a hearsay witness (who may be lying).
The possibility that this girl has been brainwashed by a manipulative socipoathic young adult, while low, should be considered until eliminated. Face it, while there are mature 14 year old girls, there are a good number who are vulnerable to manipulation, and there are sadly a few 20 year old men who are willing and able to manipulate a 14 year old into lying to protect him. Of course, the possibility of a manipulative mother or police officer or prosecutor cannot be dismissed either. In the case of the police officer, manipulating suspects is part of their job, and it's not out of the question they may use those skills on an uncooperative witness.