Hello Mary
Shelly is quite right. Both you and your wife have to consider the consequences of coming out to your friend as it can go several ways. On the one hand she may completely accept you after the revelation, and then she may not and want nothing to do with either of you again. Either way you will have to either tell her it's OK for your friend to tell others, or it's not OK, and now you put pressure on her. Also, another question you have to consider is what are you going to reveal exactly, that you are a transsexual, or a cross dresser etc...
You have said that you are still trying to come to terms with who you are, so maybe it would be best to wait a while until you are a little more certain. Once you start this journey it's heard to go back, and then after a certain point it's almost impossible. Right now you have control of the issues that you face. Once you start to come out, you will quickly loose control of those issues, and that is where it becomes tricky.
I would recommend you wait, think about what you have been told here, discuss all this with your wife and then plan, plan plan.
Chat later,
Steph