I think you need to stay out of the car business and allow him to figure things out. He got himself into this mess, therefore he needs to get himself out. You can't rescue him. He'll never grow up if you do this.
He will need to get this car business taken care of asap and stop all contact w/her. He's certainly not going to be able to negotiate w/her on the car, so he needs to contact the car company about it and advise them of the situation and that he's not going to pay any more on it. That they can come and repo it. It's a loss, but it's better to have this loss than to continue dealing w/her. The only advice I for him is to run as far and as hard as he can from her.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
