I think a good idea is at first to just say it, answer any questions they might have while keeping cool, and explaining what transgender really means, and possibly telling them about HRT.
If she doesn't quite understand, you could try telling her something like this:
"Imagine one day, you woke up as the same person you are now, but instead of being in a female body, you're in a male body. It might seem pretty interesting at first, but after the novelty wore off, you'd start to miss your old body. Not only that, but everyone now thinks that you are, and always have been, a male. They expect you to act like a male, wear male clothing, etc, and there is no way for you to change back to your old body."
Of course you may change it up a bit so it doesn't sound so direct, but I heard someone say that a while back, and I think it can give people a decent understanding of what it's like to be us. Of course, if she gets it, you don't need to say all that, but it may help. 🙂
