Sem-Cheyne was not responsible though, she said.
The evidence of three women would be critical.
They had made police statements saying they had seen a man fitting Sem-Cheyne's description kicking someone on the ground.
However, Stevens suggested they had lied to protect their friend, the real attacker.
When they each sat in the witness box on Monday, their evidence was equivocal.
Yesterday, Crown prosecutor Mike Mika agreed to offer no further evidence and Judge John Macdonald duly dismissed the charge, ending a lengthy spell on bail for Sem-Cheyne.
Cooper and his friend Joshua Henderson went to a 21st birthday party in 2018 after one of the hosts invited them at the last minute.
It was soon clear, however, that not everyone at the gathering was happy with their arrival.
Henderson was grabbed by the throat while Cooper tried to intervene.
The victim gave evidence that he was struck several times by different people before being marched out of the house.
As he walked down the driveway, he said, he heard glass smashing and footsteps behind him.
Just as Cooper reached the road he felt something impact the right side of his head.
He eventually came to while prone on the ground.
"I remember just lying there staring up at the night sky," Cooper told the court.
Henderson said he saw his friend fall between two cars – but not who hit him – before he too was chased from the scene and assaulted.
Cooper was hospitalised and became violently ill before a CT scan revealed he had sustained a complex skull fracture.
Surgeons had to remove a piece of bone that had pierced the lining of the brain, the court heard.
In the dock, Cooper pointed out the large scar running down the right side of his head.
He had been left with a slight stutter at times but said he was now "mostly well".
