@MrReed ...
Hello there. Just to confirm... when you look at the "People" page of the instructor's course, is the student enrolled in the same section as all the other students in the course (look under the "Section" column of the "People" page). Next, if the student is in a different section than the other students, go to the SpeedGrader for the assignment in question. At the top right corner of the SpeedGrader, you should see a student's name. If you click on the student's name, a drop-down menu should appear with other students listed. Just above the names, there is some text that says, "Showing:" and then the name of the section that it would be showing. Is this set to "All Sections"? I'm not sure if this would make a difference or not for the instructor, and you may have tried this already, but I just thought it might be something to look into.
Also, is this a current course that is running? Has the course ended already? I'm not sure if a course end date might play into this at all or not...but it's something to maybe also check in the course "Settings" page.
Those are a couple of my initial thoughts and ideas, but again, I may totally be wrong on both of those, so I would welcome any other feedback from Community members who have more experience grading student work than I do.
Good luck, and keep Community members posted...thanks!