My sister and I bought furniture together many years ago when we were young and stupid. Long story short, she went to rehab, stuck me with the bill, and two years later I filed for BK. Because she was the co-signer on the loan, they went after her for the full amount ($4,000) and it took us three years and an attorney to get this resolved.
So based on my personal experience, I think your son needs to consult an attorney ASAP or he will be sued along with your daughter for the balance due on the car because as a co-signer he is legally responsible for that debt. As far as his wife's credit, I don't see how that would be affected unless she signed the loan as well.