Can I send a coupon code to a customer only after they leave a review?
This is a wise decision since you are ready to give a discount only for a review. There is an option to activate a coupon code only after the review is approved and published. You can send a separate notification for the customer to know that they can now use their coupon code.
To enable this option, please navigate to System > Configuration > Smart Review Reminder > Coupon and set “Publish review to activate coupon” to “Yes”: