I found this picture with my Grandfathers photos. He was stationed in Japan during WW2 in the U.S. Army as a Doctor. On the back of the photo is the guy's name and address. How would this have happened? Were there some Japanese soldiers working with the U.S.? On the back of the picture it reads: "Mark Sekiguchi (Morihiko) Josei Bashi-dori 24 Fukuoka Kyushu Japan". My Mom never saw the photo before and doesn't remember her Dad saying anything about him.
Forgive me if I am missing something, but if your grandfather was in Japan proper, then it would have been after the war, during the Occupation, since the Japanese surrender ended the war, for all intents and purposes. During the Occupation it was quite common for Japanese to be employed by the U.S. military.
First, obviously, he wasn't stationed in Japan during the war as that was like saying he was stationed in Nazi Germany. I think the military and the government see veteran status as ending with the occupation ending, but I have no hard evidence for that. I only have anecdotal evidence from my grandpa, who served in 1946 but enlisted in 1945, and was told that he has WW2 vet status; something he rejects.
The most plausible explanation is that he was stationed in Japan during the occupation which would have had him working with Japanese soldiers or former soldiers as standard protocol given they are rebuilding the country and tending to the health of the people. This would have allowed for him to make friends with "Mark" and upon his transfer home for the end of his enlistment or tour of duty, Mark would have sent this picture with his friend. However, given that the story apparently went with your grandfather, that's the best we can do is speculate.