As a mom of twin girls as my first kids (they are now 6) I can recommend what I remember really needing.
If they are not breast feeding - formula and lots of it - it is costly and once both babies are eating 6-8 oz each feeding - a Large can of formula does not go far.
Diapers were another big thing, that we went through frequently.
A Large pack and play with a bassinet feature- as a mom of twins - in a house of stairs, we did naps downstairs so that I could keep an eye on them and did not need to bring them up and down stairs a few times each day.
Batteries, Batteries Batteries! A great gift we got was like $100 in Batteries and a box to keep them in. Between bouncy seats and swings, monitors etc, they will use them.. even if it is later. To this day, we still keep a big battery box for the kids toys.
A journal, one of the things I did not realize was the Questions the Dr would ask, and aftre a few midnight feedings, you can not remember when the last time each baby ate (how much), wet diapers etc, at least for the first few months , we kept that handy in the twins bedroom to be able to quickly jot down if Abby at 3 oz and if she had a wet diaper.
As for clothes - make sure to find out if the mom sees herself dressing them similary, identically, or not anything alike. I always am amazed that people assume all moms want to dress their same sex twins in the same clothes and we all dont! I liked to dress them in similarly colored or designed outfits, got many duplicate outfits at showers, and no one ever asked me my intentions.
Baby Einstien videos were just becoming popular and there were many days that those little video saved my sanity as the twins got a bit older!
Most importantly, make sure that she knows you and her friends are there for support and to get her out of the house when she needs it because she will and not for a trip to the grocery store, but for an hour or two over her favorite coffee or getting a manicure!