My (fictitious) e-mail to them:
Hey Friend, A few months ago I noticed that you were attempting to gain access to my device. I wondered what you were after so I browsed some porn for you. It became obvious you found this interesting as you stuck around for hours on end, watching those dirty-ass videos, while I got on with some work on another system - you were clearly enduring an enormous amount of pleasure from this. This amused me and, being the friend that I am, I couldn't help but record the scenes from your webcam. I took a quick look; they're quite horrific mate! I'm tempted to pass these recordings on to your (other) friends, colleagues, relatives, and even your wife if she's still around, or just share them online to amuse myself further - perhaps you might persuade me otherwise? So much for changing your passwords - I saw you do that! Regards, Your Friend.