I would say something or get a "thinking of you" card. I know a lot of people are afraid to do "the wrong thing" which admitedly until I had my own loss I always felt afraid of. But to me, the wrong thing is doing nothing. When I lost FB (which wasn't a miscarriage but still a loss) I appreciated people who asked me what was best to do - people who let me decide how I wanted to be treated instead of the people who made decisions for me thinking they were "sparing my feelings" and giving me "space" by saying and doing nothing or avoiding me all together b/c they had babies or children or were pregnant. I felt the best thing was when people said, "What can I do?" "What would you like?" and "Let me know" and also people who said they were sorry but acted normal too - like just came and hung out, went for walks, yoga class, took me to the beach, ate lunch together - helped me get back to living. I had a friend who said, "I'm not going to send you cards w/ smiles and flowers that say you need to 'cheer up' b/c we both know that BS won't work right now - honestly, we both know that sometimes things just suck." She said, "I'll do whatever you need: listen, let you *****, yell, cry, talk about nonsense - anything you want to do!" And that was awesome.
I've also found that saying something like, "I am sad because you are sad" works too. And just being present.
I agree with JenH too - I am sorry about your BABY. I remember my PCP saying, "You lost the pregnancy" and I retorted quickly, "I had a dead baby." Harsh. Ouch. I know. But that's what it was. My baby died. And I say loss, too, b/c I guess it sort of lessens the blow - but the reality is, the baby isn't "lost" - like missing in the woods - the baby died.
I'm sure you'll make a good choice. :-)