From the picture, it looks like the crankshaft broke behind the front
flange. I saw pictures of this same sort of failure on a 4 cyl
Lycoming on a GlasStar in BC as I recall. In that case, the
crankshaft had been recently overhauled by an engine rebuilder with
hard chrome on the front journal to build up the diameter. This is
strictly a taboo form of "repair" and would greatly weaken the
crankshaft. The resulting accident totaled the GlasStar.

I hope this guy finds his prop, and that someone knowledgeable looks
at the last engine build etc.