I think she has 2 options.
a) she pays up and keeps the car, puts the finance in her own name or whatever
b) you sell it and give her back what she put into it.
I think out of courtesy you should tell her what you plan to do and give her the option of taking the car (and the finance) off you, whilst reimbursing you for anything you have paid towards it. She should, however, get what she has put in to it back.
She hasn't behaved very well at all, but that does not mean that it's ok to withhold money or sell her car without notice. Two wrongs not making a right and all that....after I left my ex partner, he left me paying off a loan that I'd taken out for him to help because he was in mortgage arrears, stating that I "forfeited the money when I left him". Money and relationship issues are separate things IMO.