I wouldn't call him a mechanic, since mechanics never say I cant.NEED ASSISTANCE!!!!!!!
Start it, and have someone try and keep it idling while you pull one plug wire at a time, and if the engine doesn't change tune, than you found one of the crossed wires.
I think if he was smart enough to put a head gasket in, then he probably checked all the easy stuff.
He got the chain off on the install would be my guess.