From what I have read so far I think that there are already some Japanese language mods that you can install (at least according to the Japanese steam comment section of the game(日本語化可能って書いたあります)). I am not entirely sure if they cover the complete game though as I haven't used them myself and if they are also covering newer expansions. An official Japanese port would be really awesome and appreciated although I can totally get that it might not be profitable anymore at this time from a corporate perspective. Considering the text amount it might be manageable compared to text heavier game genres such as RPGS in terms of project size (in terms of text quantity and not text-in-game-importance), but hiring translators and LQA (even when hiring freelancers or outsourcing it to translation companies) will still cost a huge amount of money and you would also need to consider the additional manpower that will be necessary in terms of management.

If your friend wants to play it in Japanese it might be worth to give it a shot and look for the language mods, considering that the port would need some time anyway, but I wish you and your friend the best of luck. I love the game and it would be really cool if he could also enjoy it.