It was them who he told the truth.

It was they who he told the truth.

哪个对？

It was they who he told the truth.

用主格是没有错误的，是正式语体的用法。

主句为系表结构，be后的成分为主句的表语。人称代词作表语时，正式语体都是要用主格的,第一人称在口语里可用宾格。

另外：It was those who he told the truth.   也是对的。

以下摘自 朱垲《考试英语语法》P283）

强调主语时that后的谓语动词形式应该与被强调的主语在人称、数上保持一致，被强调的主语、宾语是人称代词一般用主格。如：

It was I that did it. 是我做的这件事。

It is he that likes swimming.  是他喜欢游泳。

It was me/I that he gave the book.  他把这本书给了我。

注： 当强调的是宾语是them时， 要改用those，如

错： Was it them whom the professor regarded with such contempt?

对： Was it those the professor regarded with such contempt?

他们是不是教授如此轻视的？

(摘自朱垲《考试英语语法》）