I am sorry. That truly sucks.
I am not familiar with your back story, and it is probably not relevant anyway, but wanted to say that upfront in case I am a bit off the mark.
What N did strikes me as incredibly childish. A lot of people have underlying childish streaks. It is unfortunate when they choose to act on them as he did. Given the fact that K is poly, it seems N enjoyed having a secret - a secret from K, and from the girls he videotaped without their permission (which is just as severe a violation of trust IMO). A power issue perhaps?
My question is how long ago did this happen? Recently? Or was it long ago enough that he may have matured since then? If recent, will he choose to grow up now? And you might ask yourself if it will help you to know his motivations, which means you need to be able to listen without interrupting him as he explains. Could be you wouldn't find the information helpful, but at this point, you really don't know why he did it. But maybe the fact that he did is all that is important. Only you know the answer to this.
I won't say this is totally impossible to fix, but it is a tough one.