The Crown case was that on the night of May 12 last year Keen had been driving around with the complainant and two others, who were later dropped off to leave the pair alone.
One of the men - the woman's best friend - made a statement to police saying she had told him she was feeling "horny" that night, just minutes before the alleged rape took place in a secluded picnic area in Mosgiel.
While in the witness box, the complainant vehemently denied she had made such a comment.
That night she sent messages to her friend, who was due to give evidence the next day.
"Why did you lie and tell interviewer that you said I was horny that night when I wasn't," she wrote, after being expressly warned against interfering with witnesses by Judge Michael Crosbie.
The man said he did not remember and that it happened last year.
"Whatever. You told them I said I was horny. I wasn't," the teen responded.
"Do you want to be my friend or not?" she asked later in their exchange.
"Yep but I want changes," the man said.
"OK then don't say stuff that's not true please xx" she messaged.
The witness admitted in court that the complainant had been in touch with him about his evidence and the court quickly adjourned so that police could extract the conversation from his cell phone.
The man stood by the comments he had made in his original police statement and said he was worried his friendship with the woman was now in jeopardy.
Anne Stevens QC promptly applied to have the charge against Keen dismissed and the Crown prosecutor Robin Bates did not oppose that course.
Before sending jurors home, Judge Crosbie described the outcome as "very unusual".
"What you have heard has undermined the trial process and the course of justice," he said.
The teenager told the ODT she "definitely" regretted her actions and had been left depressed by the result.
"It's all turned to custard. It's really brought me down," she said.
However, she stood by her opinion that her friend's evidence was incorrect.
"At the end of the day he was wrong, because he was talking crap," she said.
Although the rape charge disappeared, Keen was jailed for two years and nine months on a raft of dishonesty charges.
Keen had previously spent nine years in an Australian jail following a 2007 attempted murder. Court documents show he lured a teenager to an abandoned prison before stabbing him several times and dropping a large rock on his head.
Judge Crosbie said Keen had spent most of his adult life in prison.
