What a good friend you are!! Seriously, brought me to tears.
I think that the best thing I've heard, is what my sis said to me, "I'm so sorry sweetie that this has happened to you, I love you and don't know what else to say." That was really nice to hear and it has stuck with me. It was honest and coming from the best place. Don't pretend it didn't happen, and try not to push either, thats probably the best advice I can give. Acknowledge that it's happened and leave it open for her to discuss when she's ready.
I could provide a list of like 101 things she won't want to hear... but I'll just leave a few.
"what happened/do they know why?"
"these things happen..."
"its your bodies way of saying something wasn't right."
althought these comments are most times well meaning they really do echo, and hurt.
Good luck hun and

to your friend.
P