He took his final breaths in a gutter surrounded by police and paramedics.
Another man, who tried to stand between the crowd and Nxumalo, was also beaten to death.
Nxumalo's sister, Mbali, said the boy was known to slip into a mental state in which he would do "crazy and funny things".
"Yesterday was no different. It just happened at the wrong place and people interpreted wrongly," she said.
"Mlu was Bab'u Cele's best friend. They travelled together every morning and every afternoon, and it was not the first time they had to pick up the child at a taxi rank."
"Wednesday was his birthday and he was looking forward to it all this year, but he never got to see it. We are shattered," she said.
Cele's wife, Jabulisile, said her husband had watched helplessly as his friend was kicked to death and was shaken to the core.
"He's too traumatised. When I arrived yesterday at the scene he was paralysed with shock. His mouth was dry and his blood pressure had shot up," she said.
She said her son escaped the chaos, but Nxumalo was surrounded.
"I just don't know what to explain to Mlu's family. We were all like family, because as much as he was my husband's friend, he was much younger and we regarded him as our son. He called me 'mama' and called my husband 'baba'," she said.
Durban police are investigating two cases of murder.
'We are shattered': Man killed in mob attack was helping friend
Mlungisi Nxumalo's only crime was looking after his best friend's 11-year-old son - an act of friendship he paid for with his life.
Nxumalo was beaten to death by a mob at the Pinetown taxi rank on Tuesday evening.
He was falsely accused on the eve of his 44th birthday of trying to kidnap the mentally ill boy.
Bukisa Cele left his son in Nxumalo's care for mere moments as he went to buy food from a stall at the rank. It was then the boy had a screaming fit, drawing the attention of bystanders, and a crowd gathered.
As Nxumalo was trying to restrain the child the crowd set upon him.
He took his final breaths in a gutter surrounded by police and paramedics.
Another man, who tried to stand between the crowd and Nxumalo, was also beaten to death.
Nxumalo's sister, Mbali, said the boy was known to slip into a mental state in which he would do "crazy and funny things".
"Yesterday was no different. It just happened at the wrong place and people interpreted wrongly," she said.
"Mlu was Bab'u Cele's best friend. They travelled together every morning and every afternoon, and it was not the first time they had to pick up the child at a taxi rank."
"Wednesday was his birthday and he was looking forward to it all this year, but he never got to see it. We are shattered," she said.
Cele's wife, Jabulisile, said her husband had watched helplessly as his friend was kicked to death and was shaken to the core.
"He's too traumatised. When I arrived yesterday at the scene he was paralysed with shock. His mouth was dry and his blood pressure had shot up," she said.
She said her son escaped the chaos, but Nxumalo was surrounded.
"I just don't know what to explain to Mlu's family. We were all like family, because as much as he was my husband's friend, he was much younger and we regarded him as our son. He called me 'mama' and called my husband 'baba'," she said.
Durban police are investigating two cases of murder.