An email notification will be sent to reviewers when a business responds. There is also mobile push notifications being planned to be added at a later date.
According to Marissa Nordahl, the community manager at Google My Business:
When businesses respond to, or update responses to customer reviews, the customer now receives an email notification. The business’s response is published immediately and 5 minutes later, the notification is sent. This 5 minute delay allows time for the merchant to make any corrections to their response after submitting.
The notification email informs the customer of a reply to their review, and contains a link to a page with the full owner response.
Note: The review response is published immediately on Search and Maps.
The following screen shot is of the email notification:
If businesses become aware of the responses they leave to a review, and they know that the reviewer will become aware of that response, it could impact the type of responses they leave.