You could just do the standard red white and blue cake. Or you could look up the flag for South Africa and do a cake with both flags to represent past and present.
Although I am not sure of the circumstance of her leaving her home country, if you know and it was for reasons that she might not want to be remined of, I would say just do the red, white and blue.
A flag would be perfect. You could add the United States Oath of Citizenship either piped or printed on a frosting sheet.
http://en.wikipedia.org/wiki/Oath_of_citizenship_(United_States)
Hmmm....I think the American Flag would be a good idea too. I would be afraid of her reasons for leaving like beth2027 said. Or you could do a book of some sort that is very American...I had an idea and POOF it just left...sorry I'm not to much help!!!
I've been trying since I first saw this post to come up with something which is not stereotypical.
When a person takes the oath of citizenship here, it is often called "adopting their new country." How about adoption cake, where the citizens of the USA adopt her! Like a cake mentioning the newest member of our American family.
I know, I know - corny. Sorry, naturalization is an emotional thing for me. I can remember seeing my grandparents taking the oath. I tried.
Theresa 
How about an Uncle Sam Wants You as either an edible image or a fbct. Only as a US Citizen instead of an Army recruit. http://www.sonofthesouth.net/uncle-sam/uncle-sam-wants-you.htm
How about something with the statue of liberty (ellis island)? Something along the lines of "Give us your poor, your tired, your huddled masses longing to be free..."
What about combining BOTH flags? Using only American details seems to underline that she is giving up her country...
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