We have nothing that can provide that. Closest it gets is CB ProfileBook as it comes with a rating field (that can be displayed on a userlist), but there's no checks to limit who can post on their profilebook. Best I can suggest is use CB Privacy to limit access to the profilebook tab to connections only then have your users connect with the trainers, but then other users can't even see the reviews (should still be able to see the rating though). You may want to look into a review type extension for Joomla that can better provide that functionality.