First off, Drac, it is more than likely that if it is a genuine katana then it will be a gunto blade i.e. not something priced in the six-figure ball park.
If it is a true katana of heritage then it is also more than likely he will get little for it as the Japanese government will want it back (it gets a bit legal on such matters).
The best first step is to take it to a museum with a militaria expert on staff. He/she might not be able to appraise the object but they will be able to determine whether it is worth pursuing and also will know a-man-who-can when it comes to putting a legitimate estimate on something.
This is the part I find most amusing.If it is a true katana of heritage then it is also more than likely he will get little for it as the Japanese government will want it back (it gets a bit legal on such matters).
Interesting point, Bruno. I guess that'd be a matter for the courts to decide if it ever came to it. Even then, I can imagine that refusing to give up such a 'spoil' would be a workable tactic, at least for a while.
After all, we have a number of things in the British Museum that rightfully belong to other nations and those nations have clamoured for their return for quite some time with no sign of success.