What to say to a friend who is expecting a child and just a month before delivery her hemoglobin is 9.7 and she's worried about it?

There is probably not anything specific you can say. The infusion will make her feel a little better quite quickly. Maybe just ask her if she needs help with anything, it's likely she is super tired. Just being there for someone is sometimes enough. We don't always need the right words. Though what I would suggest with such low iron, after the baby is born, maybe give her a helping hand she is going to be even more tired.