попробуйте
$query = $this->db
->select('*')
->from('post')
->join('user', 'user.user_id = post.user_id')
->join('friend', 'friend.user_id = (post.user_id AND friend.friendship_id=1) OR (friend.user_id = 1 AND friend.friendship_id = post.user_id)')
->where('post.type', 'friend')
->order_by('postid','DESC')
->get();
$arrResult = $query->result();