Return current is confusing mostly in that it's not accurate since current, in a well functioning VAC circuit flows alternatively in both direction in direct relation to the voltage cycle. Think of it as a straw and pushing/pulling liquid through. The straw has to be able to push and pull...
He can do anything he wants. All the solutions provided have various pros and cons. My take on it is that I don't like downsizing or upsizing plugs/receptacles/breakers. It's misleading and at worst potentially life threatening if somebody else uses it or if you forget or aren't paying attention...
I'm no expert but... instead of fiddling with his plug or with his existing 6-50 outlet wiring or breaker panel, couldn't he just put together a small enclosed, plug in, grounded box that goes 6-50P > 30A double pole breaker > to 6-30R?
You plug the 6-50P end of the box in the 6-50R on your...