Dewani, 28, was shot when a taxi she was in was hijacked in the Gugulethu township in November 2010.
Her husband Shrien Dewani, 32, from Bristol, is accused of arranging her murder but denies the charge.
An extradition order for the care home owner to face trial in South Africa had been agreed, but was halted by the high court in London on mental health grounds.
Mngeni was named as an accomplice by Mziwamadoda Qwabe, who last week pleaded guilty to her murder. Qwabe was jailed for 25 years last Wednesday.