Alan,

I've faced this very issue myself.

While we can intuit that transgenderism is wrong, we have little guidance on this new phenomenon in terms of formal doctrine or teaching. Thus, there is no Catholic teaching yet to technically "break" on the question of using pronouns or names. So we have to fall back on using a judgment call.

My advice is to use a person's current legal name when addressing them (just like you would address, say, Alice Cooper or Evelyn Waugh or a boy named Sue). You will make no friends and get nothing accomplished work-wise by calling your co-worker "Ethan". You are not lying when you do this because "Luci"" is presumably now his legal name.

Pronouns are a bit more complex. They can be avoided somewhat by using his legal name in lieu of pronouns. The argument that you accomplish nothing good by using the correct pronoun still applies, although it's a bit of a weaker argument since you are making an erroneous gender assertion in using the pronoun, unlike with the legal name. I'd say if you can skate by using his legal first name, do so, praying that the pattern is not discovered. Fortunately, pronouns are typically not used in the presence of the person in question, so there may be less sensitivity that would precipitate discovery.

I have not come up with a good argument that would solve the problem of bearing false witness by using the preferred (erroneous) pronouns. That's not to say there isn't one. With due regard to the difficulty, you point out, I would point something out, and that is that if you want to draw people to Christ and be a positive witness, calling them by pronouns (or names) they do not wish to be called by is only going to seriously mar people's view of Christians. And the salvation of souls is a very critical consideration. When you refer to "Ethan", perhaps you can think of it like how a psychiatrist might treat someone who thought he was Napoleon, that is, work within the framework of his belief system (in other words, humor him even though you know he's mistaken).

Eric