One of the reasons many students quit is their instructors doesn't express an interest in them continuing.

Make the call. "Haven't seen you around here lately, when would you like to schedule your next lesson?" If the student is not interested, you can always offer to meet for lunch, coffee, or even a breath mint (this broke the ice for one student). Don't worry about the sale of CFI services, try to save the relationship. Students feel really bad when they make a simple mistake, they feel much, much worse if they ding an airplane.

Now if the school is going after the student for damages, the student has a really good reason for not being interested in further lessons. You might not be able to do anything with that situation.

Good luck!