Possible blown head gasket or worse

Coulda just blown the gasket but even if so, probalby has warped the head and perhaps the block too. If the head is cracked, then no real reason that the block can't be cracked too. So just re-working the head may or may not be a waste of time. At the very least if there is coolant in the cylinders

Your idea of the hydrolock is not all that far fetched. If it happened while running, then a rod could easily be bent; and if not a bent rod, rings or even ring lands on a piston can be busted. With the head off, rotate the engine so each piston pair is at TDC and see if they all come up evenly; use a scale to measure them closely, don't just eyeball. No way to really tell on the rings and lands without pulling them all (that I know of). If there is that much coolant in the cylinder(s), then I'd just be pulling it all out.

I kinda want to see your friend (from a distance, like observing in a zoo) and then again I kinda don't..... LOL