If the timing marks are aligned between the cam sprocket and the crankshaft, everything else should fall into place. If that guy is as inept as it seems, I'm not surprised that you had a disaster. Pistons get destroyed for two basic reasons, tight connecting rod bearings that rip the small ends of the rods out of the piston, or badly mis-adjusted ignition timing that melts the tops out of the pistons. A severe vacuum leak on a couple of cylinders will make them run lean enough to damage pistons, also.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!