Think of yourself as a Red Cross worker who is taken to the scene of a natural disaster—the author's writing—and plunked down in the midst of disorganization and good but failed intentions. You have to rely on your instincts, what your supervisor tells you, and your training. An emotional reaction to such a situation is natural, so after you read the document and react to it, put it aside. Take it out later and reread it. The second time, you'll be able to evaluate it more objectively. Do this one or twice more, and you'll be able to separate your personal reaction from your professional goal: helping the author communicate with the reader.
You are really in the service of the English language and the printed word—not that of a poor writer. If you do your work well, both you and the writer will realize this and benefit from the experience.