What do you do when a student shows up to lesson without their music books and you don’t have a copy of the music they are playing? The student not bringing their materials can be very frustrating and you are then kind of out-of-sorts if you are not prepared for this type of thing. It is disappointing when this happens.
Take advantage of this lesson to work with the student on sight reading. Everyone needs help sight reading and you can do an evaluation of where the student is and then help them build this skill.
Sight reading is: to read and perform (music) at sight, without preparation. Sight reading can only be done one time. After the first time through, the student will perform it better the next time they play it.
This will allow you the teacher, to see where they need work in music theory. Maybe you need to create a theory lesson for them. They may need to review the notes on the staff and where they are on the piano. This is also a great time to evaluate timing. Does the student skip the rests?
Be prepared in case this ever happens. You don’t want to be using their time trying to figure out what to do for the lesson. If you are prepared it can be turned into a skill building session that you may have not had any other time for.