Karen Harrison had told officials her son fell from her friend's arms, but police say video shows the mother dropping her child during a parking lot brawl.
The resignation came after Darrell Johnson said he would not leave his daughter alone with an accused pedophile, which he says was taken out of context.
Deputies found Kostial's body on Saturday morning while on routine patrol.
The woman is accused of using her boyfriend's credit card to leave the tip as revenge, and she's charged with theft in the incident.
Neighbors delivered food, water and gasoline to the man, who was with a child in a van, as ICE agents stayed at the home for hours.