Hi Jump,
I am so sorry for your friend's loss. I also had very premature twins, they both survived birth but one of our babies later died in the NICU.
You sound like a really lovely, caring friend. It is hard to give too much advice as everyone reacts very differently to situations like this but this is what I found helpful at the time.
I know you say you don't want to send a 'congratulations' and a 'sorry your baby died' card at the same time and, whilst I wouldn't put them in the same envelope, I wouldn't shy away entirely from sending both. I found it very strange when I received congratulations cards with no acknowledgement at all of my daughter who had died or of how precarious her sister's situation was. Maybe one of these would fit the bill www.redbubble.com/shop/greeting-cards+loss+of+a+twin as well as a congratulations card. It really is a time when I think I experienced every emotion going.
As john says practical help is always wonderful, cooking, cleaning and so on. Having a baby in intensive care and grieving may make it hard to keep up with all the day-to-day household stuff. Particularly if they have other children too.
This sounds strange but one of my friends used to bring me heaps of magazines! All the celebrity ones and home improvement ones, any sort of distracting stuff that doesn't require too much engagement. I loved them and looked forward to each new delivery!
If her surviving twin looks likely to be in for a long NICU stay (my survivor was in hospital for four months after we lost her sister and she had a pretty easy time of things) then gifts of snacks and nice hand creams might be appreciated. You have to wash your hands A LOT and they get cracked and painful. If your friend chooses to express milk for her baby it can be hard work and make you very hungry. DVD box set to watch if she is getting up at night to express?
And, of course, just to be a listening ear. Your friends might want to talk, or they might not, or maybe not quite yet. There are a lot of lovely Facebook groups, both for premature babies and parents who have lost a baby from a multiple birth, which might be a source of support either now or later on? It can bring up emotions, months and years down the line. Personally I found their first birthday very hard as it was so different from what I had dreamed of and imagined.
But I think a caring friend, who will both celebrate and mourn with you, is really the very best support you can have at times like this 