The judicial system is not like a game of golf where you can get a mulligan. Once the charges are brought and there is a case opened in the court, your girlfriend's best course of action now is to get an experienced defense attorney who will discuss with her all of the potential options that are in her best interest, which are not appropriate for discussion in a public forum like this.
I seriously wish both of you the best.
This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
Unless there are other witnesses to the slap, the state still needs you to testify to prove its case. Having a police officer testify as to what you may have told him on the day of the incident is hearsay, and would deny your girlfriend her constitutional right to confront her accuser. Hearsay is not admissible at trial. As such, the prosecutor must have a live witness testifying that your girlfriend slapped you in order to have any evidence against your girlfriend at trial. Good luck.