The emtions of a miscarriage are the same as with any death. Especially that of a child, and then sometimes more. There were hopes, and wishes, and and good night hugs that are gone without the first hello. Let her talk willingly about the pregnancy. Don't avoid the topic. Don't use terms like 'for the best', 'wasn't meant to be'. Let her be mad. Let her get ANGRY.
Let it be about her. Let her grieving be about the baby.
Just going over, hanging out, and watching chic flicks with lots of crying involved helps. Popcorn, with TONS of butter! Or a HUGE sundae / pint of Ben & Jerry's :-)
Maybe arrange to have dinner delivered. Maybe some groceries.
While your visiting just quietly help with a few chores, etc.
If she was far enough along where she had purchased some items. Help her create a keepsakes box. Offer to return any other items, or donate them for her to a shelter, if need be.
While the thought of a relaxing bath is nice since she doesn't have the other child, she's not really supposed to take one for a bit. Same rules apply as when you've had a vaginal delivery.
Hugs.
M.