I've torn out hairs over that that, too. It's never 'just' a moral issue; your honor is worth more than your anger. But the kind of person who'll stomp over it will also wet themselves with a gentle reminder that you have this sensitive information, without you actually compromising the confidentiality.

And remember, there is a little good in everyone, you just have to know which orifice to go in through or whether to make a new one.
- The best weapon against willful ignorance is willful stupidity.