The 36-year-old was beaten to death in a racist attack by a right-wing football hooligan in the central Italian town of Fermo, in Marche.
Amedeo Mancini, 35, was arrested on suspicion of murder on Thursday morning.
Namdi was walking with his wife, Chinyery, on Via Veneto in Fermo on Tuesday night when they suffered racial abuse. The football hooligan allegedly called Chinyery, 24, a "monkey".
That led to an altercation and Chidi was brutally beaten on the head, reportedly with a road-sign pole, until he fell to the ground.
The attack caused a brain hemorrhage that resulted in an irreversible coma. Namdi died on Wednesday.
The couple arrived in Italy last year after escaping an attack by the Islamic extremist group, Boko Haram, at a church in Nigeria in which they each lost their parents and their daughter.
Chinyery also lost her baby after the perilous Mediterranean crossing.
Prime Minister Matteo Renzi took to Twitter to condemn “hatred, racism and violence”.
Il Governo oggi a Fermo con don Vinicio e le Istituzioni locali in memoria di #Emmanuel— Matteo Renzi (@matteorenzi) July 7, 2016
Contro l'odio, il razzismo e la violenza.
The couple had been living in the area for eight months and were lodged in a shelter run by Catholic charity Caritas. Chinyery has been moved to another centre.