>>333providing that he doesn't speak any Japanese at all, it's possible to end up in facing some difficulty every time that he spends daily life in Japan and communicates with Japanese people who don't speak English, after all.
as a result, he could be left away from the society or fall into similar situation.
so I'm sure he should be even a little more fluent in Japanese, not only English, if He is going to live in Japan to become a kind of English-speaking teacher or ALT.