File: Zimbabwe's first lady Grace Mugabe is due to attend a regional leaders' summit in Pretoria on Saturday.
• Editor's note: This article is updated each time new information becomes available.
The Zimbabwean first lady has denied assaulting the model in a Sandton hotel room in August.
AfriForum will also be allowed to appeal Zimbabwe's decision.
Mugabe (52) denied allegations she attacked the 20-year-old South African model using an electrical cord at a Sandton hotel in August. Instead, the first lady claims Engels tried to attack her with a knife.
The model's mother, Debbie, dismissed the claim.
Lobby group AfriForum brought an application in the North Gauteng High Court against the Minister of International Relations and Co-operation Maite Nkoana-Mashabane for allowing Mugabe to leave the country without being prosecuted.
In papers filed in court, AfriForum acting on behalf of Engels, claimed Nkoana-Mashabane did not have the power to grant immunity to Grace Mugabe.
Nkoana-Mashabane is the first respondent in the case and AfriForum wants the wife of Zimbabwe's president Robert Mugabe to be added as the second.
The minister's decision to grant immunity resulted in a public outcry, especially after the minister of police promised Mugabe would report to law enforcement authorities.
According to Engels, an irate Grace Mugabe burst into the hotel room where she was waiting with two friends to meet Chatunga Mugabe, the first lady's son, on 13 August.
Grace Mugabe then allegedly started laying into her with an electric cable.
Photographs taken by her mother soon after the incident showed a gash to Engels's forehead.
Engels also had bruising on her thighs.