hide something in single page if current author has only one post

count_user_posts will give you the number of posts for an author. get_the_author_meta gives you the user’s ID. Note that this needs to be in The Loop to get the correct user.

if( 1 < count_user_posts( get_the_author_meta( 'ID' ) , 'post' ) ) {
    echo '<div>blah blah blah</div>';
}