D.W.
Many of the suggestions I was going to make were related to a flag or a charm bracelet with patriotic charms.
Maybe it's silly, but even a framed copy of the Constitution with an engraving of the date of her citizenship.
You could also do something small and then do something together on July 4th in celebration (if you don't each have other plans). It's my all-time favorite holiday.
Have you reached out to any of your congressmen? Senators/Representatives to see if they can sign a letter of congratulations to this friend?