And if he was fired or chose to quit the job because the employer told him to shovel snow, it would be a maybe call as to whether or not he'd be able to qualify for unemployment insurance. Sometimes, when they want to get rid of someone, an employer will actually instruct them to do something far out of their job assignment, intended to be demeaning or insulting, such as go clean out the toilet when they're an executive. This is an almost direct effort to "force a quit" and usually would result in the decision that the employee had a valid job related reason to quit = unemployment approval. Or if they were fired for refusing to do it, the employer did NOT have a valid job related reason to terminate = unemployment approval.
But I agree that it would be pretty dumb to ask an older employee maybe with health issues anyhow, who didn't regularly do anything strenuous, especially if he was a sort of walking bad attitude. Because the likely thing would be that he'd pick up the shovel, make a couple of passes, grab his back and claim he'd been injured on the job, and the company would be looking at worker's comp and the works. Which is something I've seen happen before.
But a stout person with a good attitude shouldn't object, and it would be totally legal to fire them for refusing. It's quite common for the sake of worker safety for someone to be asked to push a broom or mop up a spill or pick up trash in the work area. That's not a violation of your personal space or rights.