Hello Tatt
I know I would have wondered exactly the same in your position. Not sure how very premature your friend's baby is (maybe more than mine?) but as a mum of a recent 29+ weeker, I would definitely say yes to a card (and present if you like). Like OMDB I loved receiving cards and presents for my DS when he was still in hospital. His early arrival was totally unexpected and we were in a big state of shock, but I was really touched by all the well wishes we received, and somehow all the positive messages and congratulations made me feel more positive. It also helped us feel some of the joy and excitement which does come with a new baby, even one which is tiny and struggling in hospital, and made me feel more "normal" - like the new mum I expected to be, even when all was very stressful and uncertain.
Obviously everyone is different and it is difficult to know how your friend will be feeling. But I think it is very unlikely she will be hurt by a sincere message of congratulations/welcome and support, and she could well be hurt by hearing nothing, even if it is well-intentioned.
For what its worth, I loved clothes, toys, a momentoes box, books, anything really. And yes, chocolates, cakes and home-cooked meals were also a winner.
Hope this helps and all works out well for your friend and her little one