The woman said she was going to go tell her partner, who was sleeping in the Queen Elizabeth Park.
She and Cooper both got on their bicycles and rode towards the spot the other man was staying.
The second victim had been drinking and was passed out drunk beside his tent by the sports bowl, Judge Barry said.
When Cooper arrived, he approached the sleeping man and kicked him in the head, causing a cut to his eye and swelling around the eye socket.
The victim was already partially blind in that eye due to previous injuries, and required surgery.
According to victim impact statements, he is now completely blind in that eye.
"He speaks of having been left disfigured, depressed and bewildered," Judge Barry said.
"This was an attack on a helpless vulnerable man who was asleep."
The victim's life was "blighted" by the attack.
The first victim also described being "traumatised and angry".
In a letter provided to the court, Cooper said he was remorseful, and had let down his whānau and community.
"He set himself as a kaitiaki who would look over and after the people who were particularly living rough."
But on the day of the attack he became "enraged" when he "perceived that his efforts and his ministrations with food were disrespected".
He described the attack as "a totally over the top response to a perception that he was being insulted or belittled".
"This was ultimately blind rage, and inexcusable," Judge Barry said.
Judge Barry, who was sentencing Cooper on two charges of wounding - one with intent to injure and one with reckless disregard for safety - took into account Cooper's personal circumstances.
He had suffered a significant injury in an accident some years earlier, which left him "virtually homeless" and doing the best he could to help others in his community.
He sentenced him to two years in prison.
