On behalf of your friend, thank you for being concerned about how she might react to your news.
I agree with Nell - personally, this news is always easier for me to receive via email than via phone or in person. Email gives you the opportunity to choose your words very carefully so that you're saying exactly what you want to say, and it allows her the space to have an emotional reaction before responding to you. Email might feel awkward, but I think if you acknowledge that in your email it would help - tell her you know this news is going to be difficult for her to hear, tell her why you are emailing it instead of telling her in person, and then give her all the space and time she needs, however long it may take her to respond to you.
Trust me when I say it is entirely possible for her to be happy for you while simultaneously feeling sad and miserable for herself. It is in no way a reflection of her feelings toward or about you. It might take her some time to work through her own feelings of sadness before she is able to find her happiness for you. She might find it difficult to be around you, and that feeling may come and go throughout your pregnancy.
As your pregnancy progresses, let her decide what/how much she wants to hear or can handle. In my own experience, I didn't want to hear about it all the time, but I didn't want to feel left out either. If she asks about your pregnancy or how things are going, tell her (briefly - don't open the flood gates about it). If there's something pregnancy-related you want to talk about with her, ask her first if she feels up to it... if she says no, honor that and trust that it is not because she doesn't care about you, it is more because she's not in a good place that day. I think the more you acknowledge that you know this must be difficult for her, the easier it will be for her.
And above all, as your pregnancy progresses, no matter how bad you feel, do NOT complain to her about it. Ever. At all. Find someone else to talk to about how nauseous you are, or how constipated you are, or how uncomfortable you are, or how you can't sleep, or how you have to pee all the time, or any of it. Yes, she knows there are miserable side effects of pregnancy, and she would give her right arm to be suffering any one of them.
It may take her some time, but in the end I think she'll be very grateful that you were thoughtful enough to be concerned about her at a time that is very exciting for you.
Congratulations on your pregnancy, and my best wishes for a happy and healthy 9 months.