With the very small scale he was doing it on, I dont see any problems with it. As long as he's protected and there's no one else within a few meters, diffusion takes care of the rest. The amount he was doing would diffuse to be completely harmless within a few meters. There's mercury in the air...
This thread definitely makes me feel better, my first time out, me and my mate dug a 6" hole to the bedrock, we stood around it for a good 5 minutes wondering where the signal was coming from until we looked at our boots.