She may not be able to tell you what help she needs if she is really exhausted. Bringing food over is helpful, as are the other thing the pp mentioned. If there are vending machines in the hospital, rolls of quarters are good, a cafeteria card so she can buy meals there. If parking isn't free, paying for parking for a month.
Preemie size clothes are nice, but the babies may not yet be able to wear clothes, and they may still be in the smallest preemie sizes that are hard to find anywhere but online. A small blanket or knit cap is nice.
If you are close enough to her to buy something like this, it may help her a lot to have a good breastpump for at home. Often there is one that can be used while you are in the hospital for free. But sometimes to take one home you have to rent it. Some women have better luck with pumps that aren't hospital grade and others don't, so it might be good to have a conversation with her about it when she's up to it.