A former student told me off over email and now a university he's applying to has asked my opinion about him.

I just want to respond from the student side, since I've been in a (sort of) similar situation.

First off - your student will definitely be looking for any sign of you trying to screw him if he at all suspects you might try. He's probably even thought about what to say if it comes up. So - stick with facts.

Second - the thing with pissing contests is that everyone still gets covered in piss. Yes, it's tempting, but you really can only lose by trying to screw him over. At best you make an enemy of your student (Which could bite you). At worst you come across as trying to stir the pot and your further recommendations become suspect.

Stick to facts - did he have technical prowess in the field they want to hire him for? Do you think he has the ability to succeed? If not, can you point to things besides this email that you would consider questionable?

And it's really affected how I relate to grad students - I'm way less friendly and more guarded and don't really enjoy it very much any more.

I'm sorry the student's words hurt you, but this definitely won't be the last time in your career that someone says something like that. The best thing you can do is to evaluate if his statement had merit, determine if there's something you could do better, implement that, and move on. Allowing him to have a permanent effect on you (To the point of looking for revenge when it presented itself) only hurts you in the long run.

/r/AskAcademia Thread