That's an interesting situation NC86's student is in, her parents have
letting her conversational Japanese slip that way. My kids can now have
spontaneous simple arguments with each other in English "It's mine!"
"No, it's mine!" but I don't see us not speaking to them in Japanese
soon. I doubt my wife will ever give up speaking Japanese as they grow
up.

Insensitive people in Japan, at least those who don't know that woman,
would mistake her for or perhaps treat her as foreign, in the way that
resident Chinese and Koreans I knew who spoke like native Japanese were
treated as no different from Japanese or the way I used to pass as a
Japanese if there was no reason to present ID. After they let people
know, they were in for a flurry of questions or oohs and ahhs about
their good Japanese, however. I wonder how she'd do in a Japanese Only
establishment. And unfortunately enough, the few resident Chinese and
Koreans I have known willing to out themselves have had pretty
stereotypical lifestyles, eg owning or working in pachinko parlors,
construction companies, yakiniku restaurants, or Chinese restaurants;
attending Korean universities, or openly espousing extreme
anti-imperial or pro-Korean views. I wish I'd known more ordinary
people, say an ethnic Korean nurse or naturalized Japanese who became a
government worker.