The way relationships worked in wulin, if one has a friend in wulin, one must also respect his students as well. After all, the teacher-student relationship in wulin society (and Chinese society in general) was considered parallel to that of parent and child.
Here's a hypothetical: let's say that Mui Chiu Fung and Chan Yeun Fung were in North Beggar Hung 7 Gung's territory, killing dozens of people to practice their 9 Yum White Bone Claws, including many beggars from the Beggar's Union. In this case, could Hung 7 Gung rightfully kill Mui Chiu Fung and Chan Yeun Fung without offending his good friend East Heretic Wong Yerk See (teacher to Mui and Chan)? Or would Hung 7 Gung have his hands tied because the two evildoers happen to be students of his friend?