I am supervising a master student for his thesis in a technology field. His English is so-so and it takes quite a bit of work to correct it. I feel that correcting English is not my job, but to correct the technical aspects of the manuscript. Back when I was a student, I knew of students whose English wasn't that great and who would ask friends to correct their grammar, spelling, and wording. Can I ask my student to have his text corrected in this way—of course not the technical content! I would not consider it "somebody else's work," since it would also not count as contribution in the sense of authorship. However, I am also not sure whether it is rude to have him ask somebody else to proof-read. At my university we even have an official proofreading service (but only for PhD candidates), so I would expect it to be okay.
Edit: Good answers! Thanks. Just to clarify: The English (which is none of our's mother tongue) could be a lot worse and I had worse. With my last two students I did the mistake of correcting everything, which was a lot of work. Now I thought back and asked myself whether this is really my job. According to the answers - it actually isn't. Thanks!