ETA: Thanks for all the great advice. I am just going to tell her that the paper needs more editing than I can provide and ask her to speak to the ESL and writing centers. This post will self-destruct in a while.
So one of the PhD students who studies under the professor I work for asked me to proofread her paper. I do this a lot for his students and I am happy to do it when I have time. However this paper is so poorly written I am almost positive it is going to need to be rewritten or it will get rejected from the journal she is submitting it to. Unfortunately, I have neither the time nor the skills to do this. Also, it is against the rules of the publications for an unlisted author to do any heavy revising (like a rewrite).
I feel bad because I know it is not her fault. My boss is also a professor at a University back home in China and often bring students to come study under him here (she was one of those students). Sometimes he pressures students to write for English language journals before they are ready. He speaks English proficiently but he has other research assistants who do the bulk of the actual writing for his English language publications. She does not.
I could meet with her to discuss her paper but 1) I feel like that isn't my place since I am not her teacher, and 2) When I have talked to her in the past she has had a hard time understanding me. I told her I would get back to her tonight (via email) and I really don't know what I should say. Any advice?