Nangong Shu's words were rather disrespectful to Shi Maocheng. Of course, arguing in front of the president was also disrespectful.
However, the Nangong family had suffered a lot from the Shi family for a while now. Therefore, Nangong Shu wanted to use the rare opportunity of having the president around to warn the Shi family. Otherwise, the Shi family would think that the Nangong family was easy to bully.