In Nightjohn, what are some things Mammy and Sarny do for Alice after Waller whips her?

You will find the answer in Chapter Four of the book. Remember that, in the book, Alice is a designated 'breeder.' In the 19th century, slave breeding was the despicable practice of using fertile, young black women to increase a slave owner's labor supply. Many breeding sessions were forced upon unwilling male and female slaves; in the story, Waller has a crude breeding shed set aside for the breeding sessions.

Alice does not take well to being used as a sexual asset. The experience affects her badly, and she takes to wandering in the yard and in the white house. When Waller catches her, he is furious and proceeds to shackle her to the walls of the spring house. The cruel slave owner whips Alice until her back is torn and bleeding; then, not content with this, Waller orders Mammy to rub salt into Alice's wounds.

After the beating, Waller leaves Alice shackled. As the flies land on Alice's back, Mammy puts a cloth over her back to prevent maggots from thriving in the raw wounds. When Alice is taken down the next morning, Sarny helps Mammy clean Alice up. As Mammy rubs grease on Alice's back, Sarny holds the suffering girl's hands. The main reason Sarny has to hold on to Alice's hands is because Alice keeps pushing Mammy's hands away. The pain is so excruciating that Alice does not want to be touched, even though it is necessary.

Later, Mammy makes some root tea for Alice, but she refuses to drink it. After three days, the desperate Alice runs away.