Japanese use three alphabets, kanji, katakana and hiragana. The kanji characters derived from mandarin characters.
In Hong Kong they use Traditional Chinese characters, which is almost the same set used in Japan.
To clarify: Cantonese speakers use Traditional Chinese characters, and Hong Kong natives speak Cantonese.
Yes, and HK speaks Cantonese, which is very different from Mandarin. Not that that matters though, something like 1 in 7 or 1 in 11 Hong Kong residents speak English as their first language.
The spoken languages aren't mutually intelligible, but if you know one then the other is probably a little easier to learn. He'll probably have the time (at least, he hopes he will).
And the writing is all pretty similar (Japanese Kanji, Traditional Chinese, and Simplified).