Adrian Nel, the 40-year-old son of Meryl Knox, was shot dead in a vehicle in which he was trying to escape with his father and younger brother.
They were fleeing after dozens of people were trapped in a hotel in the city of Palma, which was attacked by militants since Wednesday.
Two Britons, an Irish national and a New Zealand national were rescued during an ongoing search and rescue operation in Palma and surrounding areas, according to a Sky source.
Rescue efforts were underway before the ambush, with at least 20 people transported to safety in helicopters, according to a private South African security company.
Ms Knox told Sky News her son Adrian, from South Africa, has come Mozambique since January 8 when he worked as a contractor in the construction of an accommodation camp.
She said: “It was so hard to find out what was really going on there.
“The helicopter evacuation was not going to take place. The insurgents had surrounded the hotel.
“The only chance they had to get away was to make a convoy.
“When they were hunting, unfortunately, the insurgents ambushed them and my son was shot.
“My husband, my youngest son and Adrian were in the only car.
“I learned on Friday evening that people had been killed while trying to leave the hotel. We did not hear who was killed.
“It wasn’t until Saturday morning that I learned that unfortunately it was Adrian. ”
Ms Knox said her husband and youngest son were not injured in the attack and “managed to escape”.
They “went somewhere in the bush” until they were rescued on Saturday morning, she added.
The two men are “too traumatized” to speak in detail about the attack and are waiting to return to South Africa with Adrian’s body, according to Ms Knox.
She described Adrian as a “cheerful person who brought love, kindness and joy to anyone and everyone he met.”
“He has many friends all over the world, his loved ones love him to pieces,” she said.
“He’s just the heart of the family – a ray of sunshine.
“When Adrian walked into a room, you were automatically happy. ”
Ms Knox said people were trapped in the hotel and “little help was coming from the Mozambique side”.
“I just feel like it could have been avoided,” she added.
“It’s not like it’s something unforeseen. It happened near Palma in the recent past.
“The insurgents actually sent flyers a short time ago announcing that they were going to attack this town.
Ms Knox said the Mozambican military was in the area to protect a gas plant and had not asked for outside help.
She added that a private security company was also operating in the area, adding: “Thank goodness we wouldn’t leave anyone there. ”
Ms Knox said a village about 12 miles from where her son was killed was attacked a few weeks ago.
“Believe me, they make sure they don’t leave anyone unharmed,” she added.
“I feel like my son shouldn’t have died. There should have been more help. ”
She claimed that members of the Mozambican army “fled and did not try to protect them”.
“I don’t see how we were told there were 100 insurgents, that an army couldn’t contain them,” Ms. Knox added.
In a direct message to the Mozambican authorities, she said: “You should have had a better army. You should have called your neighbors for help, or the international community.
“Crimes against humanity have been committed here – horrific crimes against humanity. ”
The government of Mozambique had said security forces were working to restore order in Palma, which is adjacent to gas
projects worth $ 60 billion led by major oil companies including Total.
The province of Cabo Delgado, where Palma is located, has been the target of a simmering Islamist insurgency since 2017.
The Palma attack came just hours after Total announced the resumption of work on its site after stopping them due to security concerns.