She is an American

 

Don't let anyone tell her otherwise.

She has to firmly believe in it.  If she waits for every last person she meets to agree with her on that point in order to consider herself an American, she'll never be an American.

Anyone who wants to create doubt in her mind whether she is a real American has other motives, e.g., wants to put her down in order to feel better about himslef or because he feels threatened by her in some way.

It is up to her if she wants to be an American.  Those who disagree can go to Hell.