Hi
I had hardwood stairs installed on previously carpeted stairs. I hired a carpenter instead of a hardwood installer (big mistake I am now paying for) and I had major problems with him. He chose to leave the old stringer on the staircase. He cut the old nosing off to be flush with the riser. Now there is a major gap, about 2 square in. +, between the hole he 'carved out' of the stringer when he removed the old nosing, and the new nosing. I basically fired him after he messed up on a pie-shaped stair, before I had no hardwood left to work with. Trying to clean up the mess now. Should the gap be filled with drywall putty/painted? Should a new false stringer be attached to the wall? Any advice from hardwood professionals would be appreciated.

Can you post a picture?

A Dutchman may work to fill the gap. Get a piece of wood that matches the grain and species that is the same as the stringer to fit into the hole.