I had triplets and they were born 8 weeks early. I would say ask her what she needs first. I ended up a load of wash every day, and they did not wear half the clothes they had. It just became easier to grab from the clean pile than to get the other stuff they have not worn yet out. Plus they go through them so fast they did not wear them for long. If she needs cloths, a couple of packs of onesies would be fine for her. I had mine swaddled all the time in tshirts and things like that. As the baby is a premie, she may not be going out much with the baby, so dress up cloths are not really needed. I would say a few packs of diapers. They grow out of the premie diapers fast, and you cannot buy them in bulk. I looked :). If she has a lot of premie diapers, that will be one less thing she will have to worry about. I would look into a costco or sams club membership too. I dont know how much a yearly membership costs, but I know at sams they have bulk size 1-2s boxs with 220 diapers in them. They also have the bulk size formulas too. A box of sams wipes costs 17 dollars for over 700 wipes.
I also loved the babies r us and walmart gift cards I got. I did not really know what I needed until I needed it that day, so the gift cards were lifesavers. On a more pricey object, if she is planning on pumping and has not bought one yet, go to www.hackleyhme.com. They sell the Medela pump in style advanced for 200 with free shipping. I have seen them in stores for about 350 to 400.
One more idea is to get her a hair/mani/pedi appointment. They are short appointments so she will not be out and away for long, and they help her feel better. I did not want to be away from my kids for too long, but a hair cut made me feel better in so many ways and the pedi helped with swelling after being pregnant. I am sorry this is so long, I hope this helps.