We have had similar phone calls and decisions by some of our dentists (and probably more that I haven't heard of yet).
First, he doesn't have to FIRE them; he can put them on an unpaid leave of absence, subject to recall. They will still be eligible for unemployment benefits.
As to the effect on the plan, almost definitely they will have a lower deferral, and maybe even more than just the missed weeks as they may need what they would have deferred in the weeks when they come back to recover from the weeks without full pay.
I would suggest you can deal with the possible shortage on the 1000 hours by amending the plan for this one year to use a lower hour requirement (the amount to use would take into the hours not worked because of the layoff). If he didn't hire people back, I would more than likely suggest we treat it as a partial termination and fully vest those folk. But if he plans on calling them back still, I would continue to treat them as on a LOA and not have to fully vest UNTIL client actually decides he is terminating them (not expecting to hire them back).
Lastly, if they don't have 1000 hours, why is that a 410b problem? My brain may not be functioning at 100% today, so maybe I'm missing something.