Firing employee needs to follow certain steps, warning letter being one of them. Your hu*****and didn't sign, so the process is not complete. Had the company directly fired him, that might be base for wrongful termination suit. So the company took the safe step by laying him off.
However, nowadays every employer do background checks. His ex company can still say "We laid him off because he refused to sign waring letter". That's not any better than firing.
You can test the water by applying for unemployed insurance and see if the ex employer contests.
