I assume at this point he is himself aware how eager police is to help him if he calls there over his letter being delivered to wrong address and his feelings being hurt over it.
Its illegal in here too BUT when I lived in my old apartment in shitty house full of junkies, I opened a lot debt collection letters of assholes who used to live in my apartment (one stupid fuck left lot of papers full of personal information, bank information and social security number behind kitchen cabinet) and one neighbor's letter from crime sanction department about some drug related crimes. I get why it is illegal because it would suck asshole if someone opened my letters or letters of people who aren't piece of shit junkies incapable to officially change their address, but really.. what you can do about it unless you get caught and someone is able to prove it that you opened the letter intentionally. If Yaniv doesn't even know who opened the letter, there's little you can do.