The sword can easily out price the Nambu.....His best bet... don't best guess it.. have it professionally appraised, some can be several hundreds of years old, passed down from generation to generation. You can easily get in to many thousands of dollars for a true blue age old katana. There have been instances of said bring back swords being returned to families in Japan due to respect for the family history, they were hand/custom made, as each family usually had their own design and handle. This would require a Japanese historian or person very failure with Japanese history and families to decipher the sword. Also, there is also tons of low quality/fakes that exist, this would/could be discovered and weeded out by a professional.
I have a seppuku sword brought back by my grandfather. I have thought about having it appraised, but it stays locked away.