Can I ask a Master's student to have their text proofread? 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!
As a supervisor it is not your job to proofread your student's work, but it is your job to help them produce good-quality work. Good communication skills are critical in research, therefore you should take some sort of action to improve your student's writing. When you communicate this to your student, make sure to present it in a constructive way, rather than as a criticism.

As to how you should proceed, you have a few options. One option is to ask another student in your department that has stronger English skills to help your student proofread their work. That only works if both you and your student know another student that you're both comfortable working with, and if that student has time to help. A second option is to ask your university's proofreading service if they can help your student. A third option, which may be the best bet if your student's writing needs a lot of work, is to suggest they enroll in a technical writing course to develop their skills.