Hi Dolly,
I speak Japanese. If your friend is able to give the name (by PM if you'd rather not make it public), I will see what I can dig up. If he continued in government work on his return to Japan, there may be information floating about on the internet. If they know where he came from or the original "kanji" (Chinese characters) for his name, that would help (there are often multiple ways to write names, particularly personal names). If not I will make a best guess and see if I can't find some reference to prove which is correct.
There are not the same types of bmd records in Japan - there is a system called Koseki which has a register for each family. However, it is not easy to gain access, unless you can find a relative in Japan who is willing to obtain a copy for you.
http://en.wikipedia.org/wiki/KosekiIn the law, any "直系卑属" (lineal descendant) of someone on a koseki has the right to a copy of the same, so a child or grandchild may be eligible if you have a UK birth certificate with his name on it and can show the connection down to a living descendant. I am not 100% sure if they will accept that and if the lack of marriage will be an issue, but at any rate you would first need to determine where precisely to apply as the records are held locally.
