Show a message on front-end when someone is editing the post

check.edit.post.php

<?php
function mk_check_edit_in_progress($content) {
  if (!is_single())
    return $content;
  if ( !$lock = get_post_meta( get_the_ID(), '_edit_lock', true ) )
    return false;

  $lock = explode( ':', $lock );
  $time = $lock[0];
  $user_id = isset( $lock[1] ) ? $lock[1] : get_post_meta( $post->ID, '_edit_last', true );
  $time_window = apply_filters( 'wp_check_post_lock_window', AUTOSAVE_INTERVAL * 2 );

  if ( $time && $time > time() - $time_window && $user_id != get_current_user_id() ) {
    $user = get_userdata($user_id);
    $text = 'is currently editing this post';
    $admin_defined = get_option( 'mklasens-post-edit-status-settings' );
    if (isset($admin_defined['display_text']))
      $text = $admin_defined['display_text'];
    $prepend = '<span class="mk-post-locked"><i>' . $user->user_nicename . ' ' . $text . '.</i></span>';
    return $prepend.$content;
  }
  return $content;
}
add_filter('the_content', 'mk_check_edit_in_progress');