This is where the ethics around privacy get really difficult. On the one hand Google should respect everyone's privacy, but on the other the man is a child pornagrapher.
What worries me is that once these boundaries are crossed for one very important reason, the precedent can become justification for crossing them for other less universally-acknowledged reasons.
If Google knows the content of this guy's email account based on hashes of attachments, that means they have hashes for ALL of the images in his account, and only flagged the ones that were on the list for child porn content. But I'm pretty sure the ability to identify all other content in his account by hash is laughably easy. In order to know what content isn't kosher, you have to be able to tell what content IS.
Say someone has pirated digital content in their account - an archived movie or mp3 or something. What would stop Google from coughing up the email addresses of everyone that had that particular attachment in their account in response to some zillion-dollar lawsuit by the entertainment industry?
And if your immediate response would be "but they wouldn't do that", why would you think not? Short of the morals or ethics of the human beings at the switch, what would stop them? The technology is there, and obviously working, why not use it to further advantage?