You can be his/her mentor. Go to settings and set it up. If you need help with that, let me know and I can make a detail step-by-step on how to do it. When seeing it, you can see what he/she has done along the day and see whether he/she has mastery or not on the courses he/she's taking. 

You can also set yourself up as the teacher and assign lessons with due dates.  Familiarize yourself with the courses, plot out how fast you think your student could move through them to achieve mastery, and assign a day to a week at a time.  Because KA revolves around mastery versus caring solely about grades, I don't recommend assigning lessons more than a week ahead.

Also, is this through a school (in person, hybrid or distance), as a supplement to traditional school or as a homeschool curriculum or supplement?  The amount of work that can and should be expected of a high school student varies depending on what the situation is, what the subjects are, and how much help is available to the student.

You can monitor the time she is spending in Khan Academy by having her show you her progress (bottom of the menu on the left).  You will want to focus on the exercise minutes, this will be the amount of time she spends answering questions. Total learning minutes include videos. You can also track her percent of course mastery. For both of these monitoring techniques, you could set up a goal sheet for her with rewards when she reaches different milestones.