Answer:
Liza made Bod "invisible" so that the men who were looking for him couldn't see him.
Step-by-step explanation:
When Liza entered the storeroom and saw Bod who was trying to "fade away," she helped him.
Abanazer Bolger and Tom Hustings were going to come to get him at any moment to force him to take them to the place where he had found the brooch, and maybe they would even kill him.
Liza reminded Bod that she was a dead witch so she hid him from the sight of both men and as soon as they entered the storeroom, despite looking in Bod's direction, they couldn't see him.
That is the favor that Liza did for him.