I was going to echo WoWo's reply. Check her contract....wording is usually suitably woolly re hols...eg mutually agreeable time, as agreed by management.... some such wording that gives advantage to the employer. No union rep at her place? Might be worth an ask, just for advice. If hols been OK'd, she could well have booked up to go abroad (I don't know if you have or not), therefore she could claim she cannot cancel wihtout financial loss, and so ask her employers to reimburse her, as after the official OK was given she was perfectly entitled to expect that holiday to go through, and therefore could make plans.