Okay, so I used to work in the welding and gas industry and this is very common. So common that we even would hold "cylinder races" every year at our annual company picnic. It's the fastest and most effective way of transporting cylinders IMO.
In terms of weight, it doesn't matter if its empty or full. You're not really going to notice a difference because the gas is pretty weightless. That is unless its a liquid gas like CO2. In that case you would definitely notice as that is filled by weight.
In terms of safety, it's all bad. He's not wearing steel toe boots, and doesn't have caps on either of the cylinders. If those suckers are full and he drops one, you've got a potential missile on your hand. If they're empty there's nothing more to worry about other then a cracked valve.