One of my friends grew up with his family in a Spanish-speaking country (for ~10 or so years), knowing/learning both Spanish and English, but he and his family have since moved to the United States, and started using English almost exclusively. His Spanish has since gotten a little rusty. His parents, on the other hand, learned Spanish as a second language, and have not found their skills deteriorating much despite their lack of use. (As far as I know, and for the sake of the question, both him and his parents had very similar levels of fluency.)
After a considerable period of disuse, do more-or-less native speakers of a language forget that language faster than those who learned it as a second language?
(Note: I am not so much interested in the specific scenario I have detailed above, but rather the answer to the more general question.)