Lois Pistorius, glowered at Nel on Friday and asked in Afrikaans: "Kry jy nie skaam nie? (Aren't you ashamed?)," Beeld reported.
According to the report it is unclear if Nel heard her because he was a distance away from her and had his back turned to her while he walked out of court.
She turned to the athlete's psychologist next to her and repeated the question.
Beeld reported that while closing arguments were being heard, Lois, who was sitting in the front row of the public gallery, turned around to reporters sitting behind her but her husband Arnold nudged her with his elbow and she turned around.
The State and the defence delivered their final arguments on Thursday and Friday. Judge Thokozile Masipa said she would hand down judgment on the matter on September 11.
Pistorius was charged with premeditated murder following the fatal shooting of his girlfriend model and law-graduate Reeva Steenkamp.
He shot her through a locked toilet door at his Pretoria home on Valentine's Day last year.
He claimed he mistook her for an intruder.