Add code to the header of posts by particular author

is_author() is not for determining current user, it checks whether the Author archive page is in display or not.

Your call would be to use wp_get_current_user() or get_current_user_id() (if you are comfortable with user ID) or any similar function WP have.

Example of using wp_get_current_user()

<?php
$current_user = wp_get_current_user();

if ( 'ritesh' === $current_user->user_login ) {
    // do something
}