There's no moral obligation to work without pay, even for one day, even if you've gotten "promises" that you'll be made whole. An ethical company would send him home if they didn't know they could pay him or not, whatever the reason. If they need him to work THAT DAY because of some corporate "emergency", they should pay him cash on the barrelhead or make a similar arrangement, until they can get his regular paychecks going again.

As advice to a worker in this situation, it certainly makes sense to gut it out and take the risk of doing work without pay. It's, for most people, probably a much smaller risk than risking getting "really" fired without first having another job lined up.