I'm guessing bad gas vs ethanol based gas... I put the boat away in Dec, and had zero issues this spring, maybe had half a tank in it when it went to sleep in the garage. I don't even think I treated the remaining before the winter either.
Ethanol is crap, but some people go way overboard on how much it will mess your stuff up. A good guess is that his system isn't sealed properly and somehow water got in there.