I would bet he was using the plug for back feeding the house but I am sure
there will be a dozen people jumpinhg in to say how illegal that is (It is).
You are really going to have to determine where this 240v receptacle comes
from, what size breaker it is and what size wire. Then you can figure out if it
will serve the welder you want. You may still end up changing the receptacle
(depending on the welder) but if the wire and breaker are OK it is no big deal.