The girlfriend may have been struggling with her own mental illness, but it's hard to dispute her telling him to get back in the car when he was scared. There's no question at that point that she wanted him to go through with it. She had the opportunity to stop it and get him help. Yet she was able to keep encouraging him to commit suicide. That in particular bothers me.

That being said, she is an immature young girl who may benefit from remediation. I would find other consequences more appropriate than prison, such as mental health treatment and community service.