Good question. That exact thing has happened before, and it will happen again. Hang on, because this sh*t get really complicated. I'll try to keep it simple.
The first line of defence (for the man in question) is documentation. When an insurer goes bankrupt, they must follow a set of exact procedures if they want to cover their own legal butts. They have to provide written notice of bankruptcy to all policyholders, giving them __ days (it varies by jurisdiction) to arrange coverage elsewhere.
If that fails, insurance companies are all required to carry reinsurance. Reinsurance, in very basic (very basic) terms, is insurance for the insurance provider. Reinsurance is insurance for insurance companies, and it covers things like the insurance company going bankrupt.
Finally, there is jurisdictional (state, province, territory, etc.) legislation. That legislation will say that if XYZ Mutual Insurance goes bankrupt with no possible reinsurance and no possible creditor protection, then all the other insurers in the jurisdiction (state, province, whatever) will come together and share the cost of any verified claims incurred.