I'm a Windows 10 user and I can confirm that it works much the same as it did in 7 (and 8).
First thing's first: DO NOT - I repeat - DO NOT try to install using outdated methods like Applocale if you are already using 8, 8.1, or 10. Applocale actually treats your machine as if it never changed locale at all. On top of that, you won't find it anymore on Miscosoft's site and it's not worth searching for in this day and age.
Before installing UTAU, make sure you have Japanese Locale set (and I would advise having all programs that you plan to use in the long run installed first to avoid any language confusion during installation - most installer programs will default to Japanese). If your friend isn't American or is using a European language, make sure they have Format set where the Decimal is a period instead of a comma. If they install in the default Programs (x86) folder, they'll need to adjust their security for the UTAU folder so that Users have Full Control (otherwise, they'll need to open UTAU as admin to save oto settings, etc. if they save voicebanks to UTAU/voice)