How do you comfort a friend who had a miscarriage?

I would say to simply be there when she needs you. Make sure she knows that you are available. Everyone has their own ways of grieving and recovering from such a loss. Some people needs to talk about it, some may not be ready to do so in the early stages. Give her space but let her know that she has people to turn to if she needed any support. A simple message every one or two days (not necessarily asking "how are you doing?") would be good. You can also ask her out or visit her if you think she will appreciate the company.