Pregnancy and infant loss is not something to "sweep under the rug", I believe in our culture many, many years ago we never spoke about miscarriages. Today is much different, we have support groups, blogs, facebook pages, and many resources to help us through this most difficult time.
Unfortunately not everyone educates themselves on what to say, or not to say. I though I would share this very helpful link on "Words Women Might find Hurtful After a Miscarriage"
Things that you can say and do:
1) Give a heartfelt hug and say "I'm sorry"
2) Offer to bring a meal
3) Follow up with continued communication.
A woman who has gone through this doesn't take the weekend off to recover, sometimes it takes weeks, months, even years to get through it.
4) Always keep her and her family in your prayers.
This will mean the world to her!
Blessings be with you...