Reposes the car, change the oil, and don't give it back to her until she pays for the oil change. Be firm, and tell her if she goes over 5k again, you will take the car back, and sell it.
And I don't mean it as a threat, stick to your guns, if she goes over again SELL THE CAR.
One of the conditions of her "financing" was that she would maintain it. Which she has not.