That's an even tinier slice of a very tiny pie. You're talking about maybe 0.5 to 1% of Americans who were born near the US/Mexico border who speak Spanish better than English.
According to the U.S. Census Data, only 18 million, or 5% of Americans even live along the U.S./Mexico border. Of that 18 million, 9 million or 2.5% identify as Hispanic. Of that 2.5%, about 70% identify Spanish as their native language.
I can't find how many among the 2.5% who identify as Hispanic were born in the U.S., but even if we assume they all are, this is basically a rounding error in the grand scheme of things.