Huh? They have no “japanese heritage”. The closest they have come to experiencing it is exposure to many of the artifacts/decor in our home. They have never set foot on Japanese soil, though we are planning a trip this summer so they will see where DH grew up. Oh wait-- we eat a lot of sushi and dim sum. Does that count?
Many of the college forms have changes their descriptors to say, for example, “African-American/black” because there were, for example, caucasian south africans selecting the AA designation. So, while a fine line exists between what the OP’s friend can “legally” do vs what is the right thing to do, well I would encourage my friends and family to be honest with who they identify with, in the spirit of the question.