核心问题
He will help you as far as is possible.
as 从句里为什么没有主语?
词典解答
当从句的主语it指主句内容的时候，常常是可以省略的。不仅如此，当从句的谓语动词是be的某种形式的时候，谓语动词也可以与主语it一同省略(参见if necessary)：
I shall come to see you as often as possible. 我会尽可能常常来看你。
因此，上面的句子还可以写成：
... as far as it is possible (或... as far as possible)