In my opinion, for what it's worth
is that the prob was with your mates boat. I managed a 70ft steel ketch for a guy for a while. Electolisis was a major issue on a boat of this type with numerous AC and DC systems on board. We had marine electrical engineers aboard here and abroad and it is quite a complicated topic. I think, despite what you've been told by a "marine" engineer ( a lot of marine engineers are pretty foggy on this subject) that you had an earth leak. The boat was presumably hooked up to shore power which is the major culprit. The anodes would have soon gone, then the trouble begins. We kept the ketch for months in marinas in Hong Kong. Crete, Malta, Majorca and Lisbon. None of these places are too fussy about pontoon isolation etc and we never had a problem. On the main control panel we had "Earth Leak" warning lights which would glow if we were leaking through the hull or a fitting. If it were from welding or the pontoons why did it not "eat" every boat in the marina? Sorry, but I think that it's dig deep and learn for next time.
In m opinion.