Xolile Mngeni, who was serving a life sentence for shooting Anni Dewani while she was on honeymoon in Cape Town in 2010, had been previously diagnosed with a brain tumour.
Prosecutors allege that the Briton hired Mngeni to kill his wife in a staged hijacking, but the convicted killer was unlikely to have been called as a witness against Dewani because of his illness.
One of Mngeni's accomplices, Mziwamadoda Qwabe, has already told the Cape Town High Court that Dewani's taxi driver had contacted him and told him "there was a husband who wanted his wife to be killed".
Qwabe is serving 25 years in jail for his part in the murder, while the taxi driver, Zola Tongo, was sentenced to 18 years imprisonment.
The prosecution is expected to argue that Dewani wanted Anni dead because he is a gay man who felt trapped into marriage by family pressures.
Dewani has pleaded not guilty to conspiring to murder his wife and says they were hijacked by armed men who forced him out of the vehicle at gunpoint before driving off with his wife and later shooting her.
He returned to Britain shortly after her murder and fought extradition for three years before being returned to South Africa in April.
The trial resumes on Monday.