That is not correct. The divorce settlement is an agreement between the two spouses. It is not binding on the IRS or any other third party. We have no information as to whether the attorney tried to get the loan released, but the lender is certainly not required to do so and typically would not without additional security or fees. So suing the attorney is a long shot, and can't even be done until she tries to mitigate the damage in other ways like filing for innocent spouse.
Though no guarantee of success, innocent spouse would be easy and cheap to claim based on fairness and maybe lack of knowledge. But one of the requirements is that she not benefit from the loan. That could be hard to show since the car was part of the property division, especially if she used itr during or after the marriage. At least she must resolve the problem that she was in fact relieved of a legal debt.
Meanwhile she can probably get a settlement adjustment or other judgement against him. But that would mean a private investigator would have to find him to serve papers, and she might be better off if he stayed out of her life.