No one is placing blame on Mary Jo. What is being stated is that she's shares equally the choice of having been in that car at that particular moment in time. Testing over the legal limit and having consumed enough alcohol to slow one's reaction time is 2 different concepts. The had been together all evening, from my understanding, and therefore she knew he had been drinking. And what about MaryJo's alcohol intake that evening?
Yes, the law does make many errors. However, the law cannot compensate for another's free will. She willinglu got in the car with a driver that had been drinking, it was in the wee hours of the morning, and he was a married man. Quite possible that she has some ulterior motives that clouded her better judgement, in much the same way that other young women have let the desire for fame and fortune lead them into very unwise decisions.