Hi D.,
I work in a hospital and see moms of preemies all the time. I'd suggest doing real, concrete things for her. Such as, make a meal for her that she just needs to heat up when ready. Dessert too is nice, but not necessary. If she has other children, offer to watch them one day or evening, so she doesn't need to worry about them. Maybe take the children to McDonald's, the movies, a park, Putt Putt etc. So she not only doesn't have to worry about them, but she also doesn't have to feel guilty that she's not helping them have fun at the end of summer. Rented movies and popcorn work great too and that's pretty friendly on the budget.
Now, as for your friend. Don't wait for her to call you. Call her every few days etc. It's always okay to ask about the baby. Even when the baby is having a rough day, or a rough patch of days, moms want to know you haven't forgotten about them and their babies. They want to tell you many of the details. Just be a warm, listening ear. Ask appropriate questions, and keep your judgments out of it. Just listen.
Always remember to ask, after she's had an opportunity to talk about her baby, "Now how about you? How are you doing? What can I do to support and help you out in any way?" Maybe she'll have some really creative thing you can do to help her. Or it might be as easy as, 'could you please pick up a loaf of bread and a gallon of milk when you go to the store'. We moms have the craziest things that weigh heavily on our minds.
Another thought I had is maybe she'd appreciate you straightening up her home for her. You know if you're a close enough friend to do that. Helping with errands, meals, children, just regular keep up of her life outside of the hospital with the baby, will help her a lot. But most of all, having someone ask about the baby, the other family members (don't forget that) and her, will mean the world to her.
You're such a good friend to care enough to look into it.
S.