Hi klampner,
There's a saying in trans circles that you only transition when you are willing to lose everything, 'cause you probably will. The reality is that most of us lose friends, family, careers, homes, everything, so it's a huge, huge leap to have to take to admit it to yourself, and worse yet, to come out to everybody. The fact that she's already on HRT means that she's dead serious, and is eventually going to come out, but it's scary as hell, believe me. She will need friends when she comes out, and I'm so glad for her that she has at least one.

Just be patient with her sweetie. Maybe broach the subject by discussing a trans character from TV or Katelynn from MTV's "Real Life". Give her an opening, and maybe she'll have the confidence to open up to you.
EDIT: LOL. I should read other people's posts before I make suggestions. Janet beat me to it!
Mina.