What is the effect on a supervisor if a graduate student has been found to have plagiarised?
When reading "the effect on a supervisor", the first case coming to my mind is that of Haruko Obokata, whose supervisor had to commit suicide for her misconduct. But this is not plagiarism.

It is hard to define precisely the affect on reputation. There is a case, where a PhD student, then post doc in the same group, plagiarized 11 papers. His supervisor was the co-author in all 11 papers, but it seemed not to affect his career. I'm not sure about his reputation. 

Here are the details of the case: http://carl.cs.indiana.edu/6S/plagiarism/