Mthimkhulu disappeared in April 1982 after a short political career during which he was detained, allegedly tortured and harassed by the security police.
His mother, Joyce Mthimkhulu, on Wednesday told the commission her son had been detained a number of times. Once after returning from a spell in detention he had told her how Nieuwoudt and Roelsfose had tortured him.
She has twice been gagged by court orders and prevented from testifying. However, after a full bench of the Cape Supreme Court on Wednesday found that the TRC was not required to give prior notice to alleged human rights violators, she was able to testify on Wednesday.
Nieuwoudt's attorney Francois van de Merwe on Wednesday told Sapa after her evidence that they would "be considering their options before deciding whether or not to take any action after the revelations".
"I must consult my client," he said.
Mthimkhulu gave the Commission graphic detail of how her son had described to her how he had been chained, naked, in a cold room and Nieuwoudt had trampled on his neck and body before inviting Roelofse to do the same.
They had whipped him with electric cords and pipes.
"They demanded that he speak the truth. Siphiwe said he didn't know what they wanted."
He was often rendered unconscious, suffocated with wet towels and beaten. He was later taken to a place where security police "took the comrade to make him talk".
There they had braaied meat and thrown bones to him like a dog. They had starved him for days and given him food which was inedible.
Mthimkhulu confirmed that this was the same Nieuwoudt who had recently been convicted of killing four of his colleagues in a massive car bomb blast near Motherwell.
After a six-month spell in detention - Siphiwe's last - he told her that he had been tortured day and night.
"He said to me the police have finished me this time," she said.
The young activist's words turned out to be prophetic.
Siphiwe could not walk properly or eat. He could not urinate and began to crawl "like a cat" around the house, complaining of severe pain.
After being moved to the Groote Schuur Hospital in Cape Town for treatment and tests, it was found he had been poisoned by a chemical called thallium. His body swelled up and his hair fell out.
An emotional Mthimhkulu unfolded an old handcherchief and held up a handful of her son's hair which she said she would like to hand in as evidence.
Siphiwe was moved back to Port Elizabeth's Livingstone Hospital from where he disappeared without trace.
By this time his physical condition was so bad that he was in a wheelchair.
Mthimkhulu also mentioned former security policemen Maj-Gen Nick van Rensburg and Brig Jan du Preez, who she said had been implicated by hit squad commader Dirk Coetzee in the planning of her son's murder.
She made an impassioned plea for Coetzee or hit squad member Joe Mamasela or anyone else to tell her where her son's body was buried.
"If they can show me where the bones of my child are, I will be grateful. They must say where did they take him, what did they do to him and how," she said.
The hearing continues.