The original staircase is a "Winder" Staircase, there are three treads which are called Kites, these kites are triangular in shape.
Each Kite is housed into the newel post and then housed into the main staircase string, the main string is on the left handside of the staircase as you ascend the staircase, at the point where you step from the straight flite onto the winder the string becomes huge, the oversized string is designed as such as the kite winders are very wide at this junctior and require an oversized string for support.
Your builder has cut through the kite winder string to form the new botch staircase, in doing so the kite winder string has been undermined, the kite treads may not have adequate support, but nonetheless it is definitely dangerous.
Have a chat with BC you may have to replace the entire staircase.