Fatal error: Call to undefined function db_result()

That's because db_result() no longer exists in Drupal 7. The D7 port of this module looks to be far from complete. It's a dev release, after all. :-)

I'd recommend not trying this module on D7 until there is a tagged release.

Subscribing to keep track of this thread.

I think this is better. Gets things moving again anyway.

/**
 * Get administrator e-mail address(es)
 */
function _inactive_user_admin_mail() {
  $uid = 1;
  $admin_mail = db_query('SELECT mail FROM {users} WHERE uid = :uid', array(':uid' => $uid));
  return variable_get('inactive_user_admin_email', variable_get('site_mail', $admin_mail));
}

Patch attached.

Committed