I've been through this one - So i have a bit more knowledge.
Also you have the types abit confused - transgender is mental and transexual is physical - is your question "What would you do if she used to be a man?" or "What would you do if she mentally considers herself a man, and wishes to have a sex change?"
The questions are different because in one, she plans on keeping her vagina.
My ex was the latter kind, cross-dressed talked about hormone pills and surgery, but i didn't really mind. Then again i'm very open-minded so i thought that since i don't particularly like sex anyway and that would be the only thing to alter due to all this, then I don't really mind.
Its all about how compatable you and your girlfriend are - and i don't mean Match.com compatable, i mean are you two into things that you see in each other.
I spilt with my ex for example, because i don't really like sex - and they LOVED to have it. I just felt used and unwanted so i decided we couldn't keep it together and as gently as i could, said that it couldn't work.
If it was just a mate i wouldn't care - normality is a lie anyway, none of my friends are 'normal', most are LGBT as it is. Full house actually; ones bisexual and doesn't gender define, his girlfriend is trans and wants to become a woman (born a man), I know a hardcore Duke (also trans), and i'm gay.
But yeah - its 2010 people; get with the times, Eddie Izzard said in 2000;
"By the end of the Millenium theres gonna be a lot more blokes wearing make-up and you've just gotta swing with it"