Clearly at that point he had taken it way too far. I'm not making excuses for that and I think he is definitely in the wrong. I'm just commenting on what I believe causes these kind of situations in the first place.
You can't explain writing emails impersonating someone simply as result of "chemical imbalances in the brain" (except if you reduce every kind of human behavior to that, but then the phrase "chemical imbalance" does not explain anything). Brains and their chemical imbalances are much older than the technology of emails, or even writing.