I'm trying to think of any of the gifts I got when we adopted. Most were the usual, but very thoughtful baby gifts you'd give any new mom. As my son was an international adoption, my mom also did a cross-stitch of a teddy bear carrying his new country's flag. That's still in his room.
I think what might be special is to get something that you'd get whether it was an adoption or a bio-birth. You could get your friend a little frame, if you see one with "Grandma" on it.
I think acceptance of their family as a whole family might be important. I always hated hearing "he's so lucky to have you" (like we saved him, instead built a family with him) and (since I also had a bio child after our adoption) "which one is yours?" Both! I love both my children.
Other than that, if you see something with "Grew in our hearts" as a sentiment, that might be nice and is very common about adoptions.