Can I ask a Master's student to have their text proofread?
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.