Tell them to go to Cheju, South Korea (which is truly gorgeous), Saipan or Guam instead!
Yes it is as are many places in South Korea.
But Japanese folks have a problem gong to Korea after their sordid past treatment of the Koreans and I do not understand why but the Japanese have always had a thing for Hawaii.
I lived on the edge of Seoul and worked at Yong Son for a year there and traveled as much as possible and always had a great time and was treated fabulously by the people wherever i went.