Why did Thandi's friendship with Devonne end in the novel, What We Lose?

Thandi befriends Devonne, a black woman, during her first year of college, and they initially have a very strong relationship. Devonne is very outspoken about racial issues, but she also feels insecure around the light-skinned Thandi. The friendship eventually dissolves when Devonne does not receive the same strong praise as Thandi for her final paper, primarily due to her sub-par final paper.