Adding data to options table

This is an expansion of my comment on OP:

function is_forum_subscribed($user_ID) {
  if($user_ID)
  {
    $useremail = $this->get_userdata($user_ID, 'user_email');
    $list = get_option("mf_forum_subscribers_".$this->current_forum, array());
    if(in_array($useremail, $list))
      return true;
    }
    return false;
  }
}

You can call this function like this, instead:

is_forum_subscribed($player->ID);