The alleged victim, 20-year-old Gabriella Engels, has accused Mugabe of barging into a hotel room on Sunday where Engels was waiting to meet one of Mugabe's sons, and whipping her with an electric extension cable.
On Friday a senior government source, who did not wish to be named, said there was "no way" the leader's wife would be arrested because of the diplomatic fallout that would ensue from Zimbabwe.
Engels' mother, Debbie, said she was convinced that Grace was not going to find it easy to escape the charges levelled against her.
Zimbabwe's first lady remains quiet amid assault claims
Gerrie Nel, a former state prosecutor who now works as a private prosecutor for AfriForum, an organization that primarily represents the rights of South Africa's white Afrikaner minority, said he will offer help to Engels.
The government source accepted the view widely held by legal experts that Grace Mugabe was not entitled to immunity because she was in South Africa for medical treatment, and said the government was expecting her immunity to be challenged in court.
They said her attack on Engels was due to her finding her sons "high on drugs while they were in the company of the complainant in a hotel room".
The M&G understands that advice from lawyers consulted by the South African government indicates that the nature of Mugabe's visit and the circumstances around the alleged assault disqualify the first lady from diplomatic immunity.More news: Pope prays for victims, rescue workers of Sierra Leone mudslide
Gabriella Engels at a news conference in Pretoria.
Robert Mugabe - the world's oldest sitting president - arrived in South Africa Wednesday evening.
They were told "let us talk and this can go away. So certainly it impressed me that the family said they're not interested at all", Nel said.
Ms Engels' representatives also said they had sent a letter to the government over a convention that diplomatic immunity can not be used to escape prosecution from grave crimes.More news: Weather: Showers Heading Into the Weekend (8/11 AM)
Dirco spokesperson Clayson Monyela declined to comment, referring the request to the police ministry. Enraging Zimbabwe's president by hauling his wife before the courts would only reduce its influence further, analysts said.
South Africa's justice minister, Michael Masutha, told Reuters this week he was involved in discussions over the issue.
This isn't the first time Grace's temper has gotten her into trouble overseas, though usually nosey journalists are her preferred target.
It isn't the first time Zimbabwe has dominated the regional meeting for the wrong reasons.More news: Dele Alli scores with lethal volley as Tottenham's forwards run Newcastle ragged