I'd say that it's common for TEACHING assistants to be grading papers, but not RESEARCH assistants. Although in the case where one is both the TA and RA, the lines may be a bit blurred. Whether or not that's following proper protocol i guess depends on what project the student is hired to work on as an RA (i.e. which specific grant is he/she paid out of) and what's the scope of his/her graduate program (in my school part of the course requirement is to teach a class). In any case, communication between the student and the supervisor is key to maintaining a good relationship That is, maybe the supervisor isn't aware that the student is being overworked?