There is nothing he can do. It is common knowledge the First Lady is known as Dis Grace. She built a mansion for over US$30m, known as Graceland. Dis Grace is well known in China and has several properties and a diamond cutting business in Hong Kong.
Also let us remember that RGM whilst married to his first wife, Sally who was very ill, started the affair with Grace(a junior secretary, also married to an airforce pilot) Thus a relationship based on adultery can the one ever trust the other??
AND let us not forget the other affairs of Dis Grace, Peter Pamire mysteriously died in a car accident. Business man James Makamba, run out of town.
Give a minute to the poor body guard who was present when Sabina Mugabe told Robert about the Gono Grace affair. The body guard "died" a few days later.
So what should RGM do about the new affair? Nothing, he's been cuckolded before.