“Transphobic” is used a variety of ways, and means different things to different people. What exactly do you mean? For example, are they actively hateful, or just freaked out? Is it religious bias?
I imagine they wouldn’t be your best friend if they were a terrible person, so things may not be as bad as they seem. Perhaps it would only take some genuine listening and nonjudgmental education to solve this problem. If handled carefully and respectfully, this could be a great opportunity to change someone’s outlook in a positive way.