They looked upon him as a trusted friend, as did many others he had deceived.

参考译文：正像他欺骗了很多人一样，他们也把他当作了可信赖的朋友。

还原后陈述句结构句子为：

They looked upon him as a trusted friend as many other he had deceived (did)

(备注1:did替换句子前面相同的谓语部分： looked upon him as a trusted friend.)

(备注2:第1个as为介词;第2个as为连词)

as关系代词在从句中用作主语吧，那先行词是整句还是a trusted friend？为什么后面感觉省略了什么成分？

解答：你的理解有误，以上句子不是定语从句，是一个由as引导的方式状语从句,并且使用倒装结构。he had decived 为定语从句修饰先行词many others. 

谢谢老师分析！

解答如下：

以上为 as引导得方式状语倒装结构,意思像....一样。

A:倒装结构

The book was banned in the US, as were two subsequent books. 

这本书在美国遭禁，随后出版的两本也是一样。

B:不倒装结构

I'll behave toward them as I would like to be treated.

我会以希望别人对待我的方式来对待他们。