There is something a bit wrong here. He claimed to use a "50 amp plug" because he wanted to have plenty of reserve, since he was using 30 amp circuit. We don't know all of the details. but this is the upper pole--and this is the neutral for the 50 amp system. (one poster said it was the second hot leg, (red) but that is not what the diagram for that plug shows--the white or neutral.)
So how was he wiring the 50 amp plug for a 30 amp circuit? We don't know.
Moral, wire the plug as it is intended. Be sure any connection is tight, use anti corrosion grease on the blades.
I am not commenting on the Face Book, since I use a pseudonym on Face book.
Water intrusion is possible, but this should be the upper pole of the plug, and not the one to collect water.
You cannot know exactly what happened, unless you have all of the facts, and photos of the exact wiring, how the other wiring looks, and a circuit diagram of how he did the job.