What a great idea. How wonderful that you would be so helpful to her, especially in this anti-immigrant climate we have!
If it's a standard cake, you could write something in English and Spanish, or you could write "Welcome". If you want help, let me know - don't rely on the cake decorator to get it right (Google "cake fails" to see the crazy stuff people wrote on cakes by mistake!) You could also skip the writing and do something like this American flag cake with strawberries & icing for the stripes and blueberries for the blue field of stars. http://allrecipes.com/recipe/22987/red-white-and-blue-str...
It fits in for July 4th anyway, and berries are in season, plus it will have special meaning for her.
It might be nice to show that you all honor her heritage and want to celebrate that as well. She doesn't lose that, of course, by becoming an American citizen. You could do a Mexican flag appetizer display - I had a caterer do it for my school's cultural arts festival. We had a whole year of Latino/Hispanic activities and lessons, and this was the culminating meal with performances by classes. As Spanish teacher, I was a part of the school-wide planning and found a variety of activities, recipes and crafts, and I dug around in a lot of cookbooks and so forth. There are a few versions you can Google, but I didn't see the one I got from an older cookbook. Using a rectangular tray/platter, make 3 sections in green, white and red. The green is sliced green peppers, the white is jicama slices, and the red is red pepper strips. In the middle of the white, put a circle of either black beans or black sliced olives for the emblem. I've also seen them with just dips - guacamole, sour cream and tomato salsa, with beans or olives in the center, and then with veggies or some sort of cracker or "Scoop" chip on the side. This will make it more than a July 4th party and celebrate her dual loyalties.
It might be fun to make a poster or use an easel with some of the questions from her citizenship exam. It's pretty enlightening when native-born Americans can't answer the things required of new citizens! And it will let her be the expert - another way to celebrate her achievement!
Have fun!