My best friend I knew since I was 12, I lived with her in college, so when we graduated and I recently came out to her, I was nervous as could be, we went out for lunch and my hairs short and im dressed like a dude, but she didn't really bat an eyelash cause I always been tomboyish lol She knew I broke up with my ex but not really why, so were going through lunch talking about everything except what I wanted to tell her, finally we get the bill and i just go so your my friend no matter what, she looks at me and goes o boy, what is it, of course, so then I go into well you know how I was put on birth control to even out my hormone levels cause I wasn't getting my monthly friend every month, she goes yeah, I go well what if by the summer I went to a doctor to increase those hormones, she goes estrogen? I go no the testosterone, soon as I said it, its like she got it, she goes omg what is your name going to be, can I shave your facial hair, omg I can look at you without feeling guilty when your body looks like a guy, you should start hitting that gym now, lets do this now, I'm excited. Seriously I did not expect her to be that way. If your really good friends with this chick, she should love you enough to not leave and be there to be supportive, you might just have to explain it (yes I was asked the, so are you going to have a penis question).
Far as some guys having the flat chest and looking masculine, if you didn't have the surgery yet, binders do amazing things lol Binders make me look flat, and some guys are just lucky to look more masculine then others, but T will straighten that all out, it just takes time, patience is a virtue in this case lol