A friend who left his family and is now newly married still communicates with his wife via telephone and e-mail. These communications are of such an intimate, explicit and stimulating nature that both of them sometimes peak in a climax of sexual excitement.
Is this a legitimate outcome of discussions about private matters?

If by ‘left’ you mean that the couple are divorced and thus no longer entitled to be intimate with the other, it is not appropriate for them to discuss sexual matters.
If however ‘left’ means that even though he is not living with his family, he is not divorced from his wife, all forms of intimacy between them are perfectly proper.