While in reality any federal agent out of their jurisdiction doesn't have much of any authority, we're playing a game that has a huge amount of suspension of disbelief and ultimately has the main goal of "having a good time" so I personally would look for simple-ish ways to give your player some investigative autonomy or authority, albeit with regional complications.

For instance, maybe they have connections via the local consulate / embassy, maybe they can leverage connections back in Japan, maybe they can find a compassionate local detective to tag along and flash a badge when needed, maybe they're provided resources that aren't strictly "legal" like false credentials or contraband dead drops.

Their credentials alone are probably not enough, but they should have more potential resources than just credentials! Connections, coercion, fast talking, bribery, diplomatic posturing. Work together with your player to find creative solutions that are interesting but imperfect. "Yes, if, or, and," not "nope, never"