Black to ground connection?
That is quite wrong for small 6-40A branch circuit wiring: 

Anywhere in the world, grounds can only be green, yellow/green stripe, or bare wire.    And the reverse applies: wires of those colors can only be grounds. 

Anywhere in North America, neutrals can only be white or gray. The reverse isn't true in multiconductor cables - whites can be re-tasked to be hots if neutral isn't needed. 

Hot is any other color. 

While using a black for a ground isn't a red-alert safety problem, the question is, what's he using for a hot!!!??  Using a bare ground wire for hot or neutral is downright unsafe.

Map the wiring, find both ends of a cable, and reallocate its wires to the correct usage by changing it on both ends.  

Beware: sometimes color changes like that are done as a workaround when a wire breaks.  

As Ed Beal points out, none of this applies to large 50+A circuits or to feeders and service entrances.