Prof. Nice is asked by a former student, Jason, to send a copy of a report they had worked on together. Should Nice comply? Why not? No reason is given for not sending the report: a mere question of courtesy, one would think.
We are now told that Nice doesn't like the report and doesn't much care for Jason either. But he sends the report anyway, only to discover years later that Jason has plagiarized it for his MA thesis. There is no problem here either: plagiarism should be investigated and punished. Nice must initiate an investigation through the appropriate authorities at Jason's university. As to what he could have done to prevent this from happening, there are several things. He might have earlier protected himself a bit by indicating on the report that it had copyright protection: "Not for publication. Do not quote without permission." He might assure himself that his students understand what plagiarism is and why it's wrong. He might ascertain that his university has appropriate policies in place. These are more management problems than ethical ones; the ethical point is to try to create conditions such that ethical violations such as Jason's are less likely to occur. It means not trusting people to the extent Nice would like to; but when the protections are in place, you can then be free to trust them more.