If a person obtains their girlfriends password, accesses their email account and uses the information privately with that other person to end a relationship is that a crime. If so, what are the consequences ?

Invasion of Privacy would spring to mind, but it really depends if they other party wanted to take matters further.

QUOTE by Curlyben;
Invasion of Privacy would spring to mind, but it really depends if they other party wanted to take matters further.
Further in what way

Well you have accessed a protected computer network using someone else's credentials, so that would be covered by Computer Misuse legislation.

Well you have accessed a protected computer network using someone else's credentials, so that would be covered by Computer Misuse legislation.

Practically speaking, the only thing that has been done is to confront a girlfriend with that information privately to end a relationship. Nothing further

Er no.
You need to have a read of this thread: https://www.askmehelpdesk.com/other-law/legality-accessing-someone-elses-account-507771.html

Practically speaking, the only thing that has been done is to confront a girlfriend with that information privately to end a relationship. Nothing further

Similarly, if a person read a written letter addressed to their girlfriend in the same manner, I know that would be an invasion of privacy - but what would the legal consequences be ?

Here you go:

Legally, US Federal law prohibits unauthorized access to a private network. So this would be illegal.

Practically, no prosecutor is going to spend time and money prosecuting this, without a major impact in money.

Opening another persons physical mail is another matter and somewhat more serious.