According to the Sunday Times, a group of grandchildren had driven to the store to buy alcohol. Ndaba however, said he had not consumed the beer in the parking lot, despite a photograph showing him standing behind a BMW X5 with the bottle raised to his mouth.
Another of Madiba's grandchildren, who was not part of the group, said the icon had often disciplined Ndaba and his younger brother, Mbuso - who has had a history of drug and alcohol related problems and has spent time in rehab.
At the time of the incident, Mbuso was seen buying Veuve Clicquot, a French champagne, and beer at the liquor store.
History of lung problems
Mandela passed away on 5 December at the age of 95.
The icon, who was elected South Africa's first black president after spending nearly three decades in prison, had been receiving treatment for a lung infection at his Johannesburg home since September, after three months in hospital in a critical state.
His condition deteriorated and he died following complications from the lung infection, with his family by his side.
Mandela, once a boxer, had a long history of lung problems after contracting tuberculosis while in jail on Robben Island.
His extraordinary life story, quirky sense of humour and lack of bitterness towards his former oppressors ensured global appeal for the charismatic leader.