Studying for an exam with an unprepared student
Ask yourself what you are getting out of it. If nothing, then give it up and just advise the student to study harder. 

But it is just possible that you are benefitting from the experience. Having to explain things to others firms up your own understanding in many fields. Many professors have had the experience that teaching a relatively low level course for the first time gave them a deeper understanding. Even something like, say, calculus. 

One of the most important aspects to learning is reinforcement and feedback. Depending on the other student's questions and issues, you may be getting both from the interaction. 

Think about it and also think about your best use of the time. Active learning (as in such a scenario) may be more effective than passive (reading notes, say).