She was driving the car herself, no? If you rear-end a car, and cause a fatal accident, you're 100% responsible. And you're obviously not paying attention to the fucking road and/or traffic. This a grown adult, With a fucking drivers License we're talking about. Not some poor child accidentally finding his dads gun.
She'll be responsible in the civil case, no doubt. But that doesn't automatically make it a crime. There has to be some form of criminal negligence at play (driving where she shouldn't be driving, speeding in excess of XXmph, reading text messages while driving, etc).