Thanks for the question.
I would categorize people with gender dysphoria as mentally/emotionally ill. Despite what the current trend and culture says, these people have a great internal struggle that has nothing to do with being born in the wrong body. We must be compassionate and patient with these folks as they must suffer a great deal.
What to call them is another problem. You can say to a friend who wants to be called by a new name,
"I'm not comfortable with using a different gender name, when I'm not in agreement with altering your innate gender.
It would be important to stress that you love the person as a person regardless of their gender struggles, but you are never in favor of gender reversal when science and other disciplines have told us it is not a metaphysical issue but psychological one.
Can we agree to a gender neutral nickname that makes both you and I comfortable while you work this through?"