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How do you know that the boss didn't ask the LW to prepare his personal taxes for him? That sort of thing certainly happens sometimes. But, to make the point again, we don't know. We don't know if the tax information was obtained ethically, or not (and unless you're a lawyer, you probably don't know whether or not it even matters how the information was discovered - I certainly don't). Maybe that information was included in the original letter, and is part of the text that Cary cut.
WE JUST DON'T KNOW!
So speculating and then offering advice - strongly worded advice in many cases, with dire predictions of legal penalties - on the basis of those speculations is both pointless and stupid. Cary got it right, and it's ironic that so many people are apparently disagreeing with him. This is a question of legality and professional ethics, and only a fool would give advice on either point without ALL of the information.
Not to belabor the point - oh, what the hell. Why should I be any different from anyone else here? I WILL belabor it. LW, if you broke into your boss's house and took his tax records at gunpoint, shot his dog and raped his wife into the bargain, then you could go to jail. JAIL!!! So you should destroy all those records immediately, take out your life savings, and head for the hills to lead the life of a fugitive. If you don't, you could spend the rest of your life in prison!