Letter of support for promotion to full professor from an "unusual" student A professor (currently Associate Professor) I used to collaborate with is being considered for the promotion to "full Professor". The Department asked me to write a letter supporting the professor's promotion as former student.

I am super happy to be able to support this promotion because I have a great opinion of this scholar and I genuinely think it is fully deserved, but I have few concerns:


I am just a postdoc, and usually referral letters are asked to other professors (ok, it's true I was asked as former student).
I have not been formally student of this person, neither I was enrolled in that university. I was doing my PhD in another university in the same city and just met the professor casually due to common friends and interests. The professor became my PhD co-advisor, but this was mainly to acknowledge the nice discussions and ideas exchanges we had over lunch about my work, more than a formal collaboration.
we did not publish papers together.


That told, I am wondering whether I should be really writing this letter, and, in the positive case, what kind of shape I should give to my message.
The department asked you to write because they want your opinion.  Of course they also asked others.  If they do not ask you specific questions, then just give the reasons for


  I am super happy to be able to support her/his promotion because I have a great opinion of her/him and I genuinely think s/he deserves that,  


If they do include specific questions in their letter to you, then answer those, or explicitly say "I cannot answer question 3 because...".