There is no evidence the investigator can find because it never happened and she has a history of dating older guys and she ran away with one just last year

You need to understand that her statement is evidence and that is enough to bring charges. Generally speaking a "he-said-she-said" case is not a strong one and a prosecutor would want to have additional evidence to corroborate the girl's story. But it is not necessary to have additional evidence for the prosecutor to bring charges. I'm sure he will want to assess the credibility of the girl before bringing charges. There is not much you can do until charges have been filed, but you might want to keep an attorney's number handy.