Just be there with her. Let her cry or rage or be silent or whatever, without comment, without judgement and without thinking about how uncomfortable you are being there. Just be there.

There are no comments that help. "I'm so sorry for your loss" is pretty much it. Anything like, "it's nature's way", "at least you won't be heavily pregnant in Summer", "you'll be able to try again" etc are just awful to hear. She's lost a child and she shouldn't feel like she ever needs to "get over it". She'll probably blame herself. Just hold her through it. It's not her fault, but she'll need time to accept that for herself.

Accept she probably won't be able to deal with pregnant women, baby shops, prams in the street etc for a long time. Tell her you're there for her however long it takes. You'll notice she'll start feeling better, then crash again, then up, then down... it's normal.