I have a friend who is a very experienced dive master. Not only would he never dive alone, but he wouldn't even dive on a trip with us if everyone was diving and no one would be on the boat.

It's just a risk that isn't worth taking. Many things can go wrong and if they do and you are solo, you are much more at risk of having other issues because of it.

But you are right, there's no rule. I actually believe it isn't even regulated and a dive shop could give a tank and gear rental to anyone (i.e. there's no laws preventing it) but for obvious reasons they don't.