If a witness is subpoenaed they must appear in court. If they don't take the stand they can be held in contempt of violating the court order (which is what a subpoena is - an order to appear).  This can face result in fines and even jail time. That having been said, if there are extenuating circumstances such that a witness cannot make their scheduled court date, they can be excused. Simply failing to appear is not an option.Bottom line, your girlfriend can request to be excused from tomorrow's appearance (but she will be re-scheduled to appear). Without being formally excused she risks a contempt of court charge. Right now she needs to contact the prosecuting attorney's office ASAP.