Better yet, they should've brought a fricking lawyer. I don't think a contract would cover an illegal street racing event.
Article 90 of the Japanese Civil Code
公の秩序又は善良の風俗に反する事項を目的とする法律行為は、無効とする。
which roughly translates to
"A juristic act whose purpose is against public policy or good morals is void."
(a juristic act is a general term for contracts and the like.)
Based on the wording of chapters 98-99, while the apprentice may have been in debt, he wasn't trying to defraud creditors by hiding assets or anything, so in the U.S. it's unlikely the Pantera could be claimed by creditors.
A quick search shows Japan has a similar law under the
Act on the Avoidance of Fraudulent Conveyances so im going based of U.S. law and assuming it's similar enough.
Considering it was a heart attack, not an expected death, it would be hard to prove he knew he was going to be insolvent (in this case, dead).
A good lawyer might be a bit expensive, but this is a USD$100,000 car, so I'd get a lawyer right away, especially considering the guy came to his door and threatened him.