Joshua French, who also holds British citizenship, was convicted of killing fellow Norwegian Tjostolv Moland, whose body was found at Kinshasa's Ndolo military prison on August 18.
Moland, 32, and his friend French, 31, were arrested in the Democratic Republic of Congo in 2009 and sentenced to death in June 2010 after being convicted of killing the Congolese driver of a car they had rented.
The men, both former soldiers, denied the charge and said the driver was killed by bandits. They said they had come to DR Congo to set up a security firm.
During the trial into Moland's death, the defence had tried to argue that he took his own life.
Mwila told AFP last month that her client was on the verge of a nervous breakdown, and "will lose it" if he continues to be held under current conditions.
Oslo had tried in vain to get the two men transferred to Norway, and expressed its incomprehension at the charges against French.
It said in December that a joint investigation between Congolese and Norwegian forces had concluded there was a lack of evidence against him.
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No executions have been carried out in DR Congo since President Joseph Kabila came to power in 2001, and death sentences have regularly been commuted to life imprisonment.
Penal facilities in the vast country date from Belgian colonial times and are decrepit and overcrowded.
Inmates are exposed to disease, dehydration and starvation. Rights groups report that prisoners can die of hunger or torture.