That is a quite the ethical challenge. I'm not going to pretend to have an answer or a solution to that one without giving it some serious thought. I get the arguments from both sides, as to why you should, and why you shouldn't let those affected know that the client has chosen to inform.

My initial response would be to act from a position of "your word is your bond" - your word is sacred, don't break it lightly. And you could put that persons life in danger by speaking out. Although, ironically, he may be putting people's lives in danger by informing. Like I said, a difficult one. Quite the conundrum.