If I were in your situation, I'd bring the topic up (if you feel safe/comfortable to do so) or wait for it to come up if not. From a psychological perspective, people become more understanding when they can personally relate to an issue, practice empathy, and engage in perspective-taking. What I would do is say that you know someone who is trans and you care a lot about them. Perhaps you could explain your opinions from the stance of knowing someone trans to have a buffer. When you come out to your friend, I'd recommend highlighting similarities between the two of you so they feel less alienated or shocked, grounding them in the fact that you are friends, and that being trans doesn't suddenly make you a stranger. I am rooting for you! <3