First off, beyond the perhaps less than swift things your buddy did previously, letting her leave with his car in the first place was absolutely retarded.
Second, going to work and leaving all his stuff readily accessible by her was equally dumb.
Third, sounds like the cops are really just trying to stay out of it and most likely really do not have a clue as to what is the current state of the law(s) pretaining to this situation. Tell him to contact an attorney ASAP that focuses on divorces. They should be able to actually give him correct insight as to what his/her rights are in this mess.
If your buddy needs help finding one, shoot me a pm as I’m an attorney (thankfully that does not deal with anything even remotely related to such BS) and I know a few people that would be knowledgeable with this kind of crap.