While many may argue that the foils are obvious (James/Stephen and John/Msimangu), another argument could be that the sons are foils of the fathers. Absalom Kumalo is a foil to Stephen Kumalo, and Arthur Jarvis is a foil to James Jarvis.
Both sons left their homes in Ndotsheni while both fathers chose to stay. Both sons question their lives in Ndotsheni, and while both fathers do later in the novel, the fathers decide that life is better in the village. Lastly, both sons die (coincidently, Absalom kills Arthur) due to their living in Johannesburg. Both fathers survive (again, due to their decisions to stay in Ndotsheni).
Going deeper, neither father seems to really know who his son is. Both only find out after each son's death. The sons left to find themselves, and the fathers needed to return (something neither of the sons could do) to the village (either physically or emotionally) to figure out who both they and their sons were.
Msimangu's foil could be any of the characters who choose to separate themselves from faith: Absalom, John Kumalo, and/or Gertrude Kumalo. Msimangu clings to faith, and the other characters seem to flee from it.