1994 Chevy Blazer Possible timing chain... or worse?

my sons 1994 S10 Blazer, 4.3LTR Vortec V-6. Engine started and ran fine for the first six months he drove it. Then one day he started it cold and while driving it locally (< 30mph) for less than a mile it started running very rough and losing power, he states he had to press the accelerator down almost all the way to get any power at all, within another half mile the roughness and power loss got so bad it finally stalled. After that the engine *cannot* be restarted as it will only crank about one revolution before getting "stuck" while trying to turn over. after shutting the key off and letting the engine sit a while the same scenario repeats itself. my mechanic friend took a look and said the engine is "shot" with no specific diagnosis. A friend suggested it is a slipped timing chain and could probably be fixed without replacing the entire engine. Thoughts??