Should say "She is from USA." As American can refer to many places, it is a continent. . . Well two actually. So someone from Mexico is actually American. Same reason someone from The Congo is African. I understand most of the world says people from USA are American. But they are including a lot of nations in that. And Does it really matter where a person comes from? No one asks to be born where they are born. What an induvidual does defines who they are.
you went waaay too deep into that lol. But since we're gonna go d e e p....First of all, it has become clear that American is used to refer to someone from the USA since no one wants to call them United Statesian. Second of all, American would be too vague of a term to refer to someone from the Americas since there are two completely different continents. It's like calling someone a Eurasian (I guess that might work for Russians but it doesn't sound right :/). Might as well just simply call Canadians, Americans (you know what I mean by this), Mexicans, Cubans, Jamaicans, etc North Americans. Might as well just call Colombians, Brazilians, Chileans, etc South Americans. Case closed
What about trying to use the term US citizen? I am from Spain but I know of many many Spanish speakers from America (meaning the continent) who feel that it isn't right that most English speakers call the US America and its people American. For many, this practice reflects arrogance and lack of understanding of the historical and actual meaning of the word America (this is way the USA is called USA and not simply A).
I know the English speaking world will never stop their practice of calling American to people from the US, but anyways, discussing linguistic topics is always good fun.