Normally a 50 amp RV outlet won't overload the neutral because the two 50 amp legs are 180º out of phase, so at any one instant there's never more than 50 amps from either leg. RV service also would only put that 50 amps into the neutral wire if one or both 50 amp legs were drawing their capacity amperage, which would require turning on every electrical device in the RV plus adding hair dryers, space heaters, etc.
You are correct in warning no to plug in to a power source for a welder, also a dryer or electric range outlet. In addition, your buddies actions speaks to the need of an EMS to protect the components of the RV from connection to faulty power.
I would hope TRC will repair or replace the unit, but fear since it was not a bad unit but bad power due to owner's error he might be out of luck.
I recommend you suggest a Progressive Industries EMS, they are guaranteed repair or replace for life and it would prevent damage from plugging in to a bad power source.