According to South Africa's department of international relations, Grace Mugabe didn't have immunity when she entered the country.
There is no basis in customary, conventional international law or domestic law for the spouse of a head of state to claim - as a right - some form of immunity when visiting a foreign state.
Africa should focus on the feasible reforms of the UN and de-emphasise its demand for improved representation on the Security Council voting reforms, given the complex politics around these issues.
The ‘functional immunity’ granted to UN officials made good sense when the body was founded after World War II. But as its organisational functions have expanded, so has this immunity.