I had something like that happen, only it was between a good friend of mine and an "electrician" he knew... Friend had this guy do some work because he thought I was too busy and all, this "electrician" completely chewed up a reversing motor starter I put together, as well as mucking with some other wiring, screwing up phase rotation in the shop and causing nothing but problems.......
I had a good chat with my friend and told him my thoughts on the situation. Told him flat out I was livid, and if he wants "hack N slash" to keep doing work, thats fine, he wont be seeing me around there any more... OR I come in, fix the screw ups, and not allow "hack n slash" touch a thing in there ever again..
Fortunatly, the correct choice was made.

A.D