wpDiscuz Private Comments is an easy tool to allow commenters create private comment threads. These threads will be available for website administrators, moderators and for the thread owners. Addon adds a small button next to the comment submit button to set it as private, also it adds [Private] button to parent comments to make it as private after the comment is submitted.
The buttons to set comments as private can be displayed / hidden for certain user roles in dashboard. Also, it allows setting certain user role to moderate private comments. This feature can be enabled / disabled for certain Comment Form in case you have many custom comment forms for custom post types. You need wpDiscuz Private Comments
Get Tpday wpDiscuz Private Comments.