A colleague of mine has recently asked me about an uncomfortable dilemma that he faces. Some years ago, a rather problematic student of his requested a recommendation letter. My colleague indicated that it would not be a good recommendation, but the student insisted. Accordingly, my colleague provided a not-very-supportive letter containing only completely non-disputable facts, basically saying "This student met the requirements for graduation."
My colleague has recently been approached by a potential employer of the student who wants to have an in-depth discussion about the student. Given the nature of the student, my colleague does not want to get drawn back into dealing with anything to do with them.
What would you recommend as an ethical and appropriate way to deal with this situation?