Re: Wife (nurse) had an affair (with Dr.). Expose after divorce??
I think the letter is weak and reeks of unsubstantiated revenge. If you want to send a letter to Dr. POS, send him a letter telling him what you think. If you want to send a letter to inform his employer or the licensing authorities of his unethical and illegal behaviors, send one. I don't believe one letter serves both purposes.
You can tell Dr. POS that he was unethical in preying upon a vulnerable woman who was being treated for depression, and you can say he is scum for breaking up marriages. You can tell him you know he engaged in all kinds of illegal activities. Whatever. Speak your mind. My guess is he won't care because he won't feel any threat, but you might feel better after berating him.
Such a letter would look like sour grapes to his boss or to the licensing authorities. They want specifics. He had a sexual affair with a woman he knew was married and who he knew was being treated for a mental illness. He engaged in an inappropriate sexual and romantic affair with a coworker (who could be considered a subordinate) using hospital facilities and resources (phone, computer, internet network). You know that he did this with at least one other married woman. He uses prescription and/or illegal drugs to party with these married women. Perhaps this rises to a date rape scenario. (I would bring up that specter of rape just to amp up the legal fear). You have proof of such activities.
Learn what the legalities are in your specific location of the evidence you obtained. Is there a legal way you could have obtained it? Definitely learn to say "that is irrelevant to the issue" when asked about how or where you got the info. Recording conversations may or may not be legal depending on your specific laws. Emails and other digital messages contain headers or other embedded tags which can show the path they took, which means you should be careful about providing raw copies.
If you provide text excerpts from emails or maybe some other internet based messaging, the hospital can probably go back through their servers and find the originals. This would be usable in court against him, whereas your copies might not be.
If you want to tell him he is slime on the bottom of a pile of dog poop, don't worry about proving your point. Just vent in a strong way.
If you want to expose his unethical (yes he is unethical and there should be consequences) and illegal activities you will need to unemotionally provide enough proof to get the authorities rolling. Be cautious not to implicate yourself in possible illegally obtained evidence. Cleanse data and don't get dragged into explaining where you got it.