Eek! that's a scary sight! Nice of the guy to offer the fix.
The split is caused by the drying out of wood, usually happens over the winter when heated air is very dry. Could be that it absorbed moisture in the summer then dried in the winter and cracked where there were screws or internal stresses. 6% moisture is very good for construction, but it needs to be sealed after construction to maintain stable moisture content through the seasons. A coat of oil or thin varnish is not enough. All surfaces of the box inside and out need to be entombed in urethane, imo. General Finishes makes excellent quality sealers that are easy to apply and look great. A few coats of oil Arm R Seal, or High Performance water Polyurethane. I like the ArmRSeal, it dries hard, resists scratching and the satin looks beautiful, like Ethan Allen satin lacquer, but it's easy to wipe on. Smooth out any dust bumps between coats with some steel wool.
The nice thing about plywood or MDF with veneer on top is that the box is stable to moisture variations. There is no grain that can crack all the way through the boards like real wood. Moisture variations can't crack plywood or MDF so the veneer has a stable substrate. But even veneer still needs to be sealed well, but not as critical, oil rub is OK.
Good luck
