The issue is whether or not the ex-girlfriend knew, or reasonably should have known (that is, there's effectively no way she *couldn't* have known) of the problem. If she did know and failed to disclose it, she may have committed fraud and your son may have grounds to either rescind the transaction (return car, get money back) or else seek monetary compensation (such as cost to repair). However, if she did know of the problem and it was an "as is" sale, he probably does not have grounds to recover compensation or rescind.