First, I am *so* sorry for your loss. Although I've had miscarriages in the past, I've never been in your particular situation so feel free to disregard my advice if you don't like it = ). I think all you can do is tell your friend the truth and let her feel however she will. This was *not* your fault. It is not something that you chose to have happen and you should not feel guilty or responsible. I'm sure that she will understand that this was completely out of your control. And if, for some reason, she would hold it against you, you need to know that those feelings are coming from somewhere completely unrelated to you. Sometimes when there is a loss, feelings can get mixed up and attached where they don't belong. (I was very angry at a friend of mine when she told me 2 days after a miscarriage that she was pregnant. The anger was completely misguided - I wasn't angry at her just the situation and it took me a while to realize that)
Also, remember that you've both suffered a loss and need time to be good to yourselves and grieve (you especially) so just take it slow; see what happens and take it all in stride. You can't control anything that's happened, you can only control the way you react and respond to it.
Regardless of what you decide, I hope you are able to find some peace with your decision. I'm sending you lots of love!
Peace
Krista