If she’s a dual citizen I’m guessing she didn’t sneak in. Or did she take part in the last amnesty?
She came to the USA from her Mexican company who employs her. The company does business in both Mexico and the USA. She's an accountant. She's worked for the same Mexican company for almost 30 years.
She's one of the FEW people I know or who have met who is 99% bilingual. Although she still has her Mexican accent she can understand almost 100% of English, can read and write English very well and can understand MOST JOKES in English.
Writing and understanding jokes in a foreign language are the TWO most difficult skills to acquire in a foreign language. I lived in Mexico City for two years when I was in my mid-twenties and those were the TWO skills that I never did get down very well.