Many novels are based on factual events (Primary Colors is about Bill Clinton's presidential campaign). However public figures usually can anticipate being the subject of fictional work,private individuals and specific companies usually do not. You can do it but you cannot expose real people to invasion of privacy and embarassment. You would be wise to run the text by a copyright lawyer before publication.
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(Even if you are not filing a lawsuit this information can be useful).
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Mr. Sarno is licensed to practice law in NJ and NY. His response here is not legal advice and does not create an attorney/ client relationship. The response is in the form of legal education and is intended to provide general information about the matter in question. Many times the questioner may leave out details which would make the reply unsuitable. Mr. Sarno strongly advises the questioner to confer with an attorney in their own state to acquire more information.
Godfather II by Mario Puzo was loosely based on the events of the Castro regime takeover of Cuba and the Havana gambling in the 1950s. TV episodes even today on Law and Order are based on real life situations (and a disclaimer is always stated). A wise course would be to check with a copyright attorney once your book of fiction is completed. Actual events are used all the time to be the basis of works of fiction. Check with a lawyer once you have it on paper.
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Yes, it seems you can use these writings. The authors would normally be the copyright owners of their writings, but both were sent to you as correspondence, which may affect the presumption of ownership, particularly with email.
You have the Human Resources' department's permission to use the warning they sent you (although I suppose they could argue that showing it to anyone is not the same as publishing it in a book), and depending on the nature of your friend's email, there is no reason to think that you can't do with that email as you like. Changing the names and dates for fictionalizing purposes is a good idea. While you're at it, why not ask your friend for a perpetual and unrestricted license to use the email, just in case the friend retains some ownership rights?
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.