Either a flaw in the shaft that just took 23 years to fail, assuming original shaft, or a misalignment eventually resulting in a broken shaft. Typically shafts will break right at the prop or strut, from either defects, damage from striking something, damaged/imbalanced prop vibration, etc.
Check your stringers since they are wood and may be suspect. Try tightening the engine adn transmission mount bolts into the stringers. Don't crank on them, but they should snug up and hold tight. If they are loose or spin and won't tighten up you've got rotten stringers. Also knock on the stringers along the whole length with a screwdriver, wrench, or small hammer. They should sound solid and make a sharp report, if the sound is soft/dull/muffled then you've likely got rot going on in those areas.
If the lag bolts tighten up fine then just replace the shaft and align it. How long has he had the boat and how long since last engine alignment?
If they don't tighten up, plan on doing a stringer job this winter. Check supraboats.com/bbs for plenty of info on stringer replacement and restoration of these boats.