You'd have to be pretty unlucky to have the keel fall off. It's bolted very nicely into
steel plates that are then welded to transverse
steel stringers that bolt onto the wooden ones. It's also encapsulated in fibreglass and glassed into the
hull. You could remove all of the
keel bolts and it would probably still not go anywhere. However, if you're worried about keel bolt
corrosion (galvanic or otherwise) take a good look at the keel bolts by backing off the nuts one at a time. You can do this in the
water. Just back off one nut so you can see the bolt properly where it comes through the bottom of the boat. If it's good, tighten it back up again and move on to the next one.
One of the problem i did have was with
mast step
corrosion. It's a problem that other WI36 owners have also reported (i think you have the same boat yes?). The aluminium
mast sits on an aluminium plate which is then bolted onto a mild steel I-beam using stainless bolts. The aluminium mast step plate can corrode very quickly indeed, but in my case thankfully took the brunt of the damage before i fixed it permanently.
Sounds to me like your friend had a serious
electrical leak perhaps, and had wired each of his through-hulls into the negative ground? Whatever the cause, what you've described is very rare, especially in such a short period of time.