Speaking as someone who went through this in Feb (lost DD1 at 32 weeks), I was really appreciative of cards. Got a lot of flowers too, which although well meant and very nice, just a reminder of what had happened because they all eventually died, IYSWIM? A card with a hand written message would be enough, because it will let you know you are thinking of her, without invading the "very personal" grief.
Speaking from my experience (which I really do appreciate may differ to other people), the hardest people to deal with were the ones who:
a) I didn't know very well, who couldn't think of anything to say, so came out with some stupid platitude like "how are you feeling?" - you can't say what you really feel (gutted and destroyed, how do you fucking think I'm feeling?), so you say "fine"... at which point they nod and the subject moves on the the weather...
b) The ones who ignored the subject totally - I've lost what I thought was a very good friend, because she's failed to even get in touch at all, not even via text or FB, let alone acknowledge what happened.
The best people, were the ones who said something along the lines of "I'm so sorry, I can't even begin to imagine how you are feeling." and then letting it drop, or the classic line from my DBrother (who I'm actually very close to, bless him) which was "fuck, that's a bit shit!" (not recommended in this situation, BTW!
)
Don't avoid it, (which clearly you aren't, because you are lovely enough to send a card and be thinking of your friend), but don't push for any details either. By that, let her take the lead on what she wants to tell you. DH and I have only shared DD's name with a very few select friends, and all other "arrangements" like her funeral etc, have been kept strictly private, because that's our way of dealing with it.