Well, unless I had plans of dating her (i know, you said you don't) I'd treat her the same as if a friend had come out and said he was gay. No different, 'cept I'd be a little more careful showering at the gym. It's the same person, just now you know a little more about her.
I'd just like to mention that there are different types of transexuals. There are some people born with a penis, and develop natural breasts. There are some who are born female, but with a massive clitoris. There are some real hermaphodites born with both a penis and a vagina. And there are recorded cases of people born with hardly any definable genetalia at all.
And then there are those who are born perfectly normal men or women, but aren't happy with that and get a sex change. Since most people don't have the money to pay for the whole thing in one sitting, the often have breast implants done first. Just to see how living as a female works. Or breast removal for women, but I'm not gonna pander to that PC crap. Assume the female side of the discussion.
Breast implants are much cheaper than genital reconstruction and can be undone, so that makes sense. While they're sitting in between, they can make lots of money making movies. That seems to be popular from what I see at the local video shop. There's hormone therapy to tone down the male aspects like facial hair, and the hormones do promote breast growth too.
Natural transexuals used to be a hidden group. Children with buggered up genitals would be consealed to protect them. But with the advent of cosmetic surgery, parents are forced to make decisions while the child is still young. Doctors have even been known to take that decision on themselves and reconstruct the child's privates without autorisation. There have been several court cases when parents have refused to let the doctors operate. Sounds similar to the circumcision debate, doesn't it?
Does it seem like I've researched this a bit?