You can only limit a Grader by adding them to a Section and limiting them to that section.
I don't know what the situation is you have, but some options might be to add that student to a manually created section, and then adding the Grader to that section, and limiting to interact with that section only. Then they will only see Grading for that student -- BUT I'm not sure, but I think the assignment would then have to be assigned to the student's section separately (and they would have to be removed/excused from assignments that are assigned to Everyone, or to their regular section.
