Effective tutoring for writing focuses on global issues such as organization and development, and on problem areas with clear explanations for the student. These tasks, performed correctly, will enable the student to become independent--able to proofread their work on their own more effectively each time.
As we all know, editing our own work can be the toughest part of writing; often we just don't see our mistakes. So the role of the tutor is not to "correct" the student's work and make it perfect--that is the teacher's job.
The tutor's role is to ask questions, to have the student express their thoughts orally and see how those thoughts transition to the writing, and to help the student use their resources (their notes, dictionaries and other books, online help, teachers) so that, at some point, they don't need the tutor!