I assume you are meeting up with her somewhere in person, and she is expecting to be looking for a girl? That alone could be an issue. If you are already on T and had surgery, you probably don't look anything like what she is expecting. I would suggest telling her before, at least warn her that you have changed quite a bit and then fill her in on the details later. I would just be concerned that if she is expecting a girl, and then a guy comes up to her, that she may freak out a bit. Not the best way for her to find out.
I know this sounds super cliche, but if she is really a friend, you won't lose her. Just keep in mind that you may completely blow her mind, and it may take some time for her to understand and deal with it.
