But in a decision released today by the Court of Appeal, Judges Harrison, Brown and Brewer denied the messages would have led the jury to return verdicts of not guilty to the charges of rape, injuring with intent to injure and sexual violation by unlawful sexual connection.
Mahoney, then 23, met the victim, known as Ms C, at an Armageddon Expo in the North Island a few years ago.
The victim was in character as Quinn and Mahoney was dressed as The Joker.
After the expo, the pair went back to the victim's home where she lived with her parents. He introduced himself to her mother, who was home at the time.
The victim hopped into the shower and Mahoney followed her uninvited, then raped her. The decsion noted that the mother knocked on the shower door while the pair were inside. But the victim was too frightened to call out.
"She wanted to call out to her mother for help but was scared," the decision said.
Mahoney raped the woman again in her bedroom afterwards, the Court of Appeal decision said.
The young woman told her parents, who took her to a doctor where injuries consistent with rape were recorded.
Against her father's wishes, the woman returned to the expo the next day and she and Mahoney later struck up a friendship on Facebook, initiated by Mahoney.
It was those messages that formed the basis of the appeal, heard in September this year.
In them, the victim said she liked Mahoney, that he made her laugh and smile and feel happy.
"... your [sic] the greatest friend I ever had".
"I love hanging out with you also. I feel special around you."
Mahoney's lawyer pointed out that it wasn't until Mahoney rejected the victim several months later saying that he wanted to be just friends, that she complained to police about the rape in May and also one in late October or early November at Mahoney's house.
But the court agreed her Facebook communication was consistent with her "autistic obsession" with Harley Quinn and other comic book characters and by extension with Mahoney.
It pointed out that Ms C only complained to police after an altercation with Mahoney in a public place when he pushed her to the ground and a constable questioned her further.
The court said although Ms C's response to begin a friendship with Mahoney after he raped her was surprising, this could be explained by her autism including that she has difficulty with the concept of friendship, has behavioural problems including depression and a tendency to internalise anger.
"We are not satisfied the new evidence might reasonably have impugned the jury's acceptance of Ms C's credibility.
"The jury was aware of her diminished life skills, impaired sense of friendship, general social incompetence and obvious vulnerability."
It also said Mahoney's treatment of Ms C, which during the first rape was particularly violent, was consistent with two other complainants who said Mahoney lifted them by the throat and strangled them to the point of almost blacking out.
The issue of consent was challenged by Mahoney's lawyer who said the Facebook exchanges proved Ms C's sexual acts with Mahoney were consensual.
But the court said the exchanges occurred after the first sexual activity and initially she was a reactive and non-committal participant in the messages.
The court said the evidence suggested Ms C's obsession with Mahoney subsided as quickly as it developed and there was nothing to show it endured after late August.
Part of the appeal over the length of sentence also failed after the court said although an offender may be eligible for credit for time spent on bail with restrictive electronic monitoring, it was a question of whether the end sentence was manifestly excessive or wrong.
Mahoney was given credit for his young age, the fact he had no other convictions, and because he suffered attention deficit hyperactivity disorder.
But the end sentence of 10 years was "well within range, if not generous to Mr Mahoney".
