He'll probably be in the hospital for 2-3 months, so you may want to think about gifts that would be nice during that time.
Her son cannot wear clothes now, although he may be able to wear them in about a month. I really enjoyed dressing our little one in his first little (tiny!) outfits. Made everything feel a little more normal. Another gift that could be used in a few months would be a little baby towel set and some nice bath products. Getting to the first bath is a big milestone, but the darn hospital towels are scratchy!
Another idea would be a blanket that could be used now as an isolette cover and later as a swaddle blanket. Or a small, soft cuddle blanket that can be used for kangaroo care and that could fit in the isolette. Someday I hope to set up an Etsy shop to make and sell these things...at this point I've made them for my own preemies and for friends.
The mom might enjoy a soft robe to use for kangaroo care - or even a care package full of healthy snacks, magazines or books, soft socks, etc. to have at the hospital. You could tuck a gift card for a local restaurant or coffee place in there too.
There are special clothes designed for breastfeeding and kangaroo care. Our hospital sold these: http://kangaroocareclothing.com/Scripts/Default.asp
and also had them available for moms in the NICU.
I've also really enjoyed the Moby wrap with my preemies...although we didn't use it until the final days before leaving the hospital, I felt like this was essential when we got home to keep the baby close (and keep germy strangers away).