If you are really close, you might want to make a scrap book to commemorate this short but significant life they shared. From pregnancy to birth, and then a few months down the road give it to your friend when she's having an empty arms day. If they have a yard that can accomodate a tree, get a flowering tree, like a pink dogwood or an ornamental cherry, a magnolia and plant it as a living memorial to that life.
Every parent handles their grief differently. My parents lost my sister when she was 4 months old to SIDS. And growing up I knew all about my sister. We took flowers to the cemetary on her birthday, Memorial Day, Easter, Christmas, she was a member of the family that was never forgotten. Years later, my brother died in an accident, he was 14... and the void was so much greater, the hurt endless. It wasn't so much what people said, it was their being there, to visit, to sit, to occupy the time and space. Take your cues from them. What do they want to do. If you attended the funeral or memorial, they know that you care. A card in the mail, thinking of you today... not a sympathy card. Go back to your routines before pregnancy stuff. Watch your words, don't say you can have another... because you can't have another one. You can create a different baby, a 2nd child, but not this child. Just hug your friend when she needs one. Be there to listen to her, her pain and her laughter. You can't take her pain away, but you can support her while she walks this most difficult of life's journeys.
Thank you for being a good friend.