restrict-wordpress-user-to-one-page-without-plugin.php

<?php
add_action('admin_init','HR_user_lockout');
function HR_user_lockout() {
  global $current_user, $menu, $submenu;
  $restricted_user = 1;
  // check for current user
  if ($current_user->data->ID == $restricted_user) {
    // remove admin menus
    $menu = array();
    $submenu = array();
    // redirect user to only permitted page
    add_action('current_screen','HR_user_redirect');
  }
}
function HR_user_redirect() {
  $screen = get_current_screen();
  $permitted = 159;
  if (
  // EDIT PAGE
  (
  $screen->base == 'post' && $screen->id == 'page' &&
    (
    (!isset($_POST['post_ID']) && $_GET['post'] == $permitted) ||
    (isset($_POST['post_ID']) && $_POST['post_ID'] == $permitted)
    )
  )
  OR
  // MEDIA UPLOAD
  (
    $screen->base == 'upload' && $screen->id == 'upload' ||
    $screen->base == 'media' && $screen->id == 'media' ||
    $screen->base == 'async-upload' && $screen->id == 'async-upload'
  )
  ) {
    // HR user is in the right places
  } else {
    // redirect to permitted page
    wp_redirect(get_bloginfo('wpurl').'/wp-admin/post.php?post='.$permitted.'&action=edit');
  }
}
?>