K.B.
I honestly think the best thing you can do is go spend some time with them. They are going through a grieving process and would need the shoulder to cry on, the ear to bend, the person to help with mundane tasks like laundry and dinner. While you're spending time with them, some ideas might pop up of things you can do that would be meaningful for them.
I think sometimes people who lose children are left alone to sort through their feelings because people don't want to intrude. Losing them after they are born is a world apart from losing them through miscarriage. Having lost one myself after birth, I can tell you that the friends I remember and am grateful for the most are the ones that rallied around me and were just physically there. Even if you're out of town, go for a visit.