A friend of mine (from the US, native speaker) spent a year in Japan as a schoolteacher under the JET program. We kept in touch via message-boards and such. His

A friend of mine (from the US, native speaker) spent a year in Japan as a schoolteacher under the JET program. We kept in touch via message-boards and such. His written English notably deteriorated over the course of the year. It's possible your friend picked up some non-native language habits during the course of his stay.