Getting back to the original work,
Why did the "whatever" in the bottom end of the engine need tightening?
I can understand torquing the rods and mains as part of reassembly after some repair or replacement work where they were previously loosened.
But I can't understand re-torquing them on a used engine just for the fun of it.
Perhaps he over did it on the "re-torque" and overstretched a rod bolt or stripped the threads out of a nut, perhaps he did actually cause the failure.
What did he actually do under there, or what was he asked to do?
It may be time for a new mechanic, a good one, and have him carryout the tear-down and inspection while the old "mechanic" watches with his hands in his pockets.
Grigg
