Consider the facts - they were probably BOTH drunk. She happened to be the one behind the wheel, but they're both equally responsible. And in some places by law (and ethically in places where not the case), wherever and whoever she had been drinking with shoulders some of the responsibility as well. Not to mention the sentence "she killed him" is a little inaccurate. In Japan, a country with 0% alcohol tolerance for driving, everyone in the car with someone who has been drinking is equally responsible for the driver. Everyone - equally. We are a little biased towards our own way of thinking. Understandable, but worth noting there are other ways of viewing the same situation.
That said, this girl deserves the 6 months, but I see no reason to punish her beyond that. It could just as easily have been her boyfriend behind the wheel that night. I've had friends die in drunk driving accidents, and I am just as angry/upset at the ones who let the person drive.