Their lawyers told the court this week the trio did not witness Sims sexually violate the other girl, who was so traumatised from the assault that she defecated.
The court heard yesterday that Kerr texted two friends to come over to the flat because there was a "chick fight" and one of them was naked.
Kerr, who was in a relationship with Sims at the time, told police in an interview that the girls started arguing but he told them he did not want to be involved in their "crap".
He said he watched Sims punch and kick the teen but just "ignored it".
"Hannah was attacking her. No guy wants to really get involved in a female fight, so I just left it and stuff happened."
He had watched Sims rub her crotch in the teen's face but insists he did not see the sexual violation. He did, however, hit her with a spatula and blow flour in her face.
"It wasn't intentional to hurt her, it was just a joke."
Kerr said he did feel bad that he turned his back on some of what happened.
"I wish I did ring you guys [police] but I didn't want to get myself in the shit."
A neighbour confronted the group that night when he heard a girl had been beaten up.
He demanded to see the girl to make sure she was all right.
He told the court the girl was "escorted out".
"She didn't come out alone, wilfully," the neighbour said.
The girl had looked traumatised. She was "shaking, hunched over ... pretty ugly, really". He asked if she wanted a lift home but she was incoherent, he said in evidence.
"I spoke to her and got nothing back." She turned her head slightly but it was like "no one was home".
She had responded only when Roper told her to go inside.
Crown solicitor Steve Manning suggested that the neighbour did all he could to help but he responded: "Looking back, I don't think I did enough."
The trial continues today.