Writing a recommendation letter for a student I reported for academic dishonesty A few years ago, I reported a student for academic dishonesty (in a graduate-level mathematics course, they were copying solutions off the internet, nearly verbatim). After acknowledging their wrong-doing with the Academic Integrity office on campus, they were allowed to drop the course. 

The next year, the student re-enrolled in my course, and did reasonably well. I had no qualms about the originality of their work this time. 

They recently asked me whether I would be willing to write them a recommendation letter for PhD programs in mathematics. 

If I write a letter, am I obligated to write about the academic dishonesty incident? It appears to me that the student has already faced the consequences of their actions (they had to drop the course); however, this was a significant part of my interactions with the student and it seems dishonest to not mention it.

(I am aware that I could simply tell the student that I am not comfortable writing a letter, but I am curious to know what one should do in this situation if one did write a letter.)

Some facts: when the incident occured, the student was in their first semester of our masters program. They are also an international student, and this was their first semester in the US. 



ETA: Look, I framed the question to be as general as possible so that it can be useful to the community as a whole. Since some commentors are choosing to attack my teaching practices, below are some more specifics, where once again I am trying to not reveal the identity of this student, myself, the course, the university, etc. I am actively trying to do what's best for the student here (note that I clearly state that I think the student has faced consequences for their actions; I am also not using my usual ac.se account to post this), and I don't understand where comments about "shutting down a student's career" are coming from. 

I clearly state in my syllabus and on the first day of class that copying solutions off the internet (or any written source) is not allowed. I do this because I encountered this situation as a TA in graduate school. In particular, students are allowed to talk to anyone they like about problems, but I believe that when they just copy a solution, they are not learning. You are welcome to disagree with my policy, but it is my policy and my class, and it was clearly stated in two separate venues.

The copied solutions were virtually indistinguishable from the solutions online. This was not a matter of simply "being inspired". This was not isolated - three out of six solutions were copied, and those were just the ones I spotted. When I noticed this, I asked the student to come talk to me. I told them that I had noticed similarities between their work and solutions I had found online. I told them that I was not accusing them of anything, and they did not have to tell me anything; I reiterated that copying solutions from the internet was not permitted; I invited them to come to my office hours to talk about future problem sets; but that if I noticed such similarities again I would report them to Academic Integrity. They did it again on the next problem set (four out of six solutions). 

Although they denied everything once via email, after talking with Academic Integrity (where they owned up to everything), they did find me and apologize. I was cordial to them throughout this event, and continued to be so afterward. We remain on good terms now. I consciously try to ensure that I do not let this incident color my interactions and behavior with them. They made a mistake once but people make mistakes and I am trying hard not to take it personally.  

In summary, the incident itself was pretty blatant, the policy was clearly stated, and they had an opportunity to stop such that I would not have reported them if they had. I did not take reporting the student lightly. Nonetheless, I think academic dishonesty is a serious matter and it is our responsibility as faculty members to not turn a blind eye to it.
The best option is:  Don't write the letter.  In a case like this, I would outright refuse to provide a recommendation.

However, if you are not comfortable with a non-negotiable refusal, you might want to simply tell the student that you cannot provide a positive letter for them.  If the student insists, then you should write an honest letter that gives a balanced evaluation of their ability and personal traits.  The fact that you caught the student cheating is a critically important piece of information in this regard, and it needs to be included. The cheating will almost certainly be the most salient feature to anyone who reads the letter, which will make the net effect of the letter, regardless of whatever else you say in it, strongly negative.  And if the student insists on you writing the letter in this situation, that is basically what they deserve.