From personal anecdata, I can assure you it's entirely possible to 'lose' a language ability. Native tongue? Not so sure, but a closely-related one, definitely!
I'm a native Dutch speaker and used to be relatively fluent in German (which is not a given: despite being close neighbors, the languages are very different). Then, I lived in Cape Town for a while, and had to learn some Afrikaans (also closely related to Dutch, but yet widely dissimilar).
This somehow 'erased' my ability to speak German! Only after moving back to Europe and after many years, I was able to do basic stuff like ordering in restaurants in German again.
I'm a native Dutch speaker and used to be relatively fluent in German (which is not a given: despite being close neighbors, the languages are very different). Then, I lived in Cape Town for a while, and had to learn some Afrikaans (also closely related to Dutch, but yet widely dissimilar).
This somehow 'erased' my ability to speak German! Only after moving back to Europe and after many years, I was able to do basic stuff like ordering in restaurants in German again.
TL;DR: the human brain is, like, weird, man...