Like I said. He DOES NOT NEED TO DO THAT.
And we aren't talking about modern charging systems here. But for a guy in a parking lot without any tools, that's a damn good way to tell if your alternator isn't working. NOTWITHSTANDING the dangers of Battery acid vapor ignition from the sparking that WILL occur when the cable is taken off and put back on a running engine with a WORKING alternator!
If the ALT is NOT WORKING, then there WILL NOT be a spark... But that's not the point.
Like I said above, and this stands true here, Listen to old SPARKLESS SPARKY (yeah I done some unconventional schitt, but I never melted a wire nor blew a battery in all my time turning wrenches, and that's a long damn time, and I did it all without one DAY of classroom teaching) Test with a multimeter a running engine AT THE BATTERY for the voltages that your vehicle is rated for. If it's not found, THEN you can go prodding around the Dangerous areas at the back of the alternator with a running engine if you want to, but I dont' recommend it. I personally do all tests after that with the engine off, and or I shut off engine and connect extension leads that will make it unnecessary to be behind the alternator with the engine running.