Trans people

[ameerkam]

Salaam

Would it be a sin (or even kufr) to refer to a person who identifies as trans by their preferred pronoun (they, ze etc.)

Jazzakallahu Khair

[Zzz_]

wa'salaams,

by doing so you would be approving of their deviance and falsehood. I would think you are not allowed to do that for the same reason you can congratulate the kuffars on their holidays, like christmas.

[Mandy]

A person is either male or female. That is how Allah created all of us.
Your friend is confused enough as it is. I do no think it is helping him/her to give encouragements and validation to that action of attempting to denying the gender he/she was created with.