This is so bizarre that you would even have this question!

The reason that legal and other privileged communications, as well as internal corporate news announcements in publicly held companies that come before external earnings calls have that awfully long and harsh sounding advisory that explains that the communications are intended for the recipients ONLY and may not be forwarded is because otherwise you are free to forward whatever you want!

And the T has a duty of confidentiality so the T would not disclose anything unless person B expressly announces his or her suicidal or homicidal plans.

I can see how you might owe a duty of extra privacy protection to an underage child who cannot insert the "awfully long and harsh sounding advisory" mentioned above... maybe. But still, T is confidential. Forwarding emails is the most convenient way to communicate with T's who agree to communicate via email and I do it all the time. Plus, it saves trees (unlike printing out). But since in your hypothetical, person B is already in an intimate relationship, that rules out the possibility that person B is a minor.

The same goes for emails to attorney, as Murray pointed out. The communications with attorneys are private, too.

How would the T proceed without knowing about third parties in your life? Any time you are discussing anything beyond the strictly internal process of self-discovery, you are discussing your relationships with third parties, and to enable you to discuss such relationships, the T's are obligated to keep the facts about third parties confidential (there are other reasons for their duty to protect your privacy as well).

Since you are asking for best practices and industry standards...

... in corporations, the standard for corporate email content is the following:

- If you do not want to see your email printed on the front page of the NYT some day, do not send it.

Private email is different from corporate email, but since you are forwarding person B's emails to the T and not to the NYT's editorial board, you should be OK. Oh, and it is actually OK to forward it to the NYT's editorial board because NYT's internal standards would prevent the NYT from publishing a private email. So you are fine.