What is the religious ruling on accessing a female friend's email that was created for her, then closing it without her knowledge, and how can sins be expiated after repentance?

There is no doubt about the seriousness of the relationship between girls that reaches the point of adoration and forbidden acts, as it is one of the gates of Satan. You have done well by repenting and cutting off the relationship. As for the email, it is not permissible to access it or view its contents without the owner's permission, even if you were the one who created it for her, as it involves a kind of espionage. It is also not permissible to delete the email entirely without her permission. If it contains objectionable material, you should advise her to delete it. If she complies, then praise be to Allah. Otherwise, you may delete it if doing so does not lead to a greater harm. You must repent from these actions, and part of complete repentance is to seek her forgiveness. If you fear harm, then supplicate for her well-being.