She needs to get a solicitor involved, really to deal with ALL their assets. It doesn't matter if the registration document is in his name - the registered keeper is not necessarily the owner.
http://www.taxdisc.di...questions/en_Help.jsp
If the car was bought during their marriage then it is normally classed as a common asset (i.e. they both own it), just as the one that he took with him would be. If he reported it stolen, knowing full well that she had it, he could certainly be prosecuted for providing a false statement and wasting police time.
If he's not going to play nicely and come to a mutually agreed settlement, then she needs professional advice to fight her corner and make sure he doesn't take everything.