I am so sorry for you and your friend's loss. Some people take it much harder than others; only you will know to what extent she will/can handle it.
I didn't have a total miscarriage, but I lost one of my triplets at 10 weeks, later went on to deliver the two surviving babies at 26 weeks gestation. It was hard for most people to recognize them as triplets so I eventually learned just to call them twins. To me, I felt as if I was simply forgetting about the third baby by calling my two babies "twins". 11 years later, I am OK with it.
There is definitely a grieving process. And, yes - you want to be there and do all the right things, and say all the right things. Most of all, your friend will probably just want your shoulder and an ear at first- don't take it personally. Eventually once she has accepted the miscarriage, the conversations will evolve into something that you can better participate in. Definitely don't dismiss her feelings, or try to comfort her by saying it was meant to be, or it was God's will, or whatever. That made me so angry to hear! (Along with the standard "Well, at least you still have the other two!" UGH! I still lost one, remember?)
Just offer to spend a few hours with her at home, and bring a home made meal or home made lunch. Maybe take her out if she feels up to eating out, and it's in your budget. A change in scenery was always a nice distraction for me. I had one friend just come over and fold laundry with me one afternoon! About a week later, she also went with me to the grocery store- it took us about 2 1/2 hours to get groceries that night but I so appreciated the company.
Mail her a nice card with a handwritten note stating you're sorry for her loss and want to contact her in a day or so. Coordinate it with her husband. Maybe you can accompany her to her follow up doctor's appointment. Just *BE* there; she will work itself though it all. Knowing she has friends who care so deeply will make it easier for her.