Being his friend for almost a year has taught me so much. He's taught me about transgenders and what not to say around him. He's forgiven me for so many mistakes I have done of which I'm still very sorry -for calling him "her". I'm thankful and grateful for his friendship.
I thought for the sake of others in my situation I'd write a short guide for you and your friend.
I feel like for starters you have to talk about it a little. When they come out to you is vital. At that moment there isn't really any point to ask questions like "Do you want me to call you him or her?" Those sorts are obvious. Asking "Are you going to change your name?" can be an OK question but it varies from person to person. No matter what make sure you're positive that they understand that no matter what gender they decide to be you will be their friend. That you'll be there for them. If you're religious, don't be at that moment. It's not needed nor wanted at that time. Refrain from preaching. That sorta thing hurts.
You also have to truly be there for them. When someone makes fun of them stand up. People have said of me "Oh god don't talk about _____ in front of DJ she'll kill you" and I honestly don't care. Cause I want them to know that I won't stand for it. For my handsome friend there is one guy who wont leave well enough alone. He uses my friends old name "Jessica" and refuses to use his current name. His reason being "She doesn't have a penis. Until she does I will call her Jessica." A completely ignorant reason. He's not the only one though. Many do it to him. It causes my head to hurt and my blood to boil. They call him a friend then disrespect him in that way! What the hell?! Fight for your friends. Don't drag them down.
If you are a teacher I highly recommend that you have ways of helping your transgender students. One of my teachers on the first day ask "When I call out your name please tell me what you want me to call you" and I think that's wonderful for transgender students. Sadly though my art teacher -whom also did that- would not allow my friend to use his name "Jakobi" when turning in artwork. Reason why is still unknown to me. Also to teachers try -if possible- to refrain from using he and she when you aren't sure. It helps. It really does.
If you are a parent I understand your dream of having a little girl is being taken away from you. I understand that you are upset. I get that. But you have a choice. You can make your child feel worthless, unloved, and unnatural OR you can be there for them. You can raise them up like a parent is supposed to. If you are confused on what to do well there are online forums when you can talk to other parents of other transgender youth. I've found this site and I find that it might give some help: 13 Tips To Your Transgender Child
In short love them just as much as you did as the gender they were born as. What they have in their pants is irrelevant. If you care about them you will be there for them. No matter gender they are the same person you knew/know. I hope this was at least somewhat helpful to those who are getting used to the new boy/girl they used to know as a girl/boy.