If you want a view a student's timetable and you are a coordinator, navigate to https://rooster.universiteitleiden.nl/admin. On this page, you will see a list of users where you can find your student based on their student number.
You can then click 'Show personal timetable' to view the timetable of this student.
With this functionality, you are only able to see the student's enrolled courses, and not the courses that the student has added themselves.
Ticking 'Show personal timetable' means that your own personal timetable will be shown at the same time.
You can also choose to add the student's timetable to your own timetable, by clicking 'Add to my timetable'. When using this button, courses from this student timetable will be added to your own timetable and you will be able to consult them through https://rooster.universiteitleiden.nl/schedule.