Question

Someone threw a cardboard box into the trash chute in our building and jammed the vacuum-compactor. The repair costs about two thousand shekels. The prohibition against throwing cardboard in there, and the reason for it, are posted and known to everyone. No one has admitted to it, so the repair is coming out of the shared building fund. As it happens, I know whose box it was, and he is my neighbor and also a friend. We were at their home for Hanukkah candle-lighting, and in a photo we took you can see part of the box, and that part remained intact when it was pulled out of the vacuum-compactor.  
Now I’m torn about what to do. What I’d most like is not to know at all that it was him, and now just to forget about it and not say anything to him. I’m uncomfortable with this, and it’s not worth the money to me—especially since I only know it “thanks” to the fact that we were guests in their home. On the other hand, it’s not fair that because I’m uncomfortable, the rest of the neighbors should get stuck with it. On the third hand, why should I have to lose out—my comfort, and maybe even the basic friendly relationship between our families—for the sake of the other neighbors? And I’m certainly not willing to pay the whole amount myself just to spare myself the discomfort, because it’s a lot of money. It seems pretty clear to me that the fairest and most mature thing to do would be to show him the proof and demand that he admit it, and otherwise I’ll report him. But that is exactly what I really do not want to do: not to show him that I know, not to demand that he confess, not to threaten to report him, and not to actually report him.
Is there any obligation on me here?