thanks for your legal question which I answer as follows:
Generally your girlfriend, as you say, could have committed the criminal offenses of forgery and fraud.
You tell me that your girlfriend used the old semester ticket to create a new one. The ticket represents a document because it contains a statement that is suitable and intended for the proof in legal relations. For it is the legitimacy that allows you to move with the train.
She is also expected to have committed a fraud. By deception with the aid of fake the ticket your girlfriend excited the false impression that she had paid the price for the right to take the train. In this way she obtained a pecuniary advantage by which she was enriched .
Your girlfriend should actually not have made any statement to the police, so that her rights could be perfectly represented. Now she obviously revealed her criminal offense, so that a legal defense ist difficult.
Furthermore there is no obligation to follow that citation of the police. Only a citation from the courts or the public prosecutor you have to obey. If you are not married or engaged with your girlfriend, it is unfortunately not permitted by law to refuse your statement as witness at the courts or the prosecutor.
I hope I could help you. Naturally you can use the unique free demand function if anything is unclear, so that I can clear out this.