I don't think you can get detained for not extending your VOA in time... she gets 30 days and leaves after 36, she will just say she preferred paying 3000thb at the airport then 2000thb extension + return trip to/from immigration. "If you are stopped by the Police" in one of those 5 days, just say you were/are planning to extend it but didn't have a chance yet. Thai police understands really well that 3000>2000.