My wife and I have suffered through two miscarriages. You can't really understand it until you experience it. The best thing you can do for her is SUPPORT her! She thinks it is her fault, that she is less of a women. She probably hasn't told you that. Reassure her. Whatever you do, never blame her, and don't let her second guess herself. "Maybe if I hadn't walked up the stairs so fast, or if I had only slept more and eaten better." Those are the kinds of irrational thoughts that a women who has miscarried will feel.
She feels a sense of profound loss that us men can never begin to understand. Take the loss you feel right now and multiply it my a million.
I hope the best for both of you!