What is the prescribed Islamic legal stance to be taken by a member of a PhD dissertation examination committee who discovers that the researcher has quoted from a previous study without attribution? Should he reject the defense, or accept it and be more stringent in the evaluation after the defense?

If a student violates the rules for doctoral dissertations, then what the regulations require must be done, along with advising the student first. If he does not respond, do not take action against him that would harm him as long as you can prevent the harm. Otherwise, choose the lesser of two harms that clarifies his mistake and serves as a deterrent, due to the Prophet's (PBUH) saying: "There should be neither harming nor reciprocating harm."