He had at last been allowed to send a fax in which he informed the editor that he had been arrested while ... 等于：

He had at last been allowed to send a fax，and in the fax he informed the editor that he had been arrested while ...

这样的句子很多。例如：

He sent me a letter in which he told me that... 他寄信给我，在信中他说...

inform 可以接“双宾语”，直接宾语是 that 从句：

inform sb. that （从句）