The best way to support multilingual/ESL students with grammar/language concerns is to develop a support model that includes yourself, your peer mentor (if applicable), and other CTLR tutors.
The first step is to talk with the student(s) to learn more about what challenges were faced on the assignment. Did they understand the assignment (and/or the course readings it references)? Did they know what they wanted to say? Did they allocate enough time for completing and reviewing the assignment?