Police said Quentin Wilson, 19, was taken into custody at a Stockbridge, Ga., Waffle House restaurant after 50-year-old Rex Leo became violently ill from the poisonous drink.
Leo was transported to a hospital, where he remains in critical condition Thursday.
"What did this poison do to his system?" Leo's sister, Linda, said. "It was acid, it ate through his esophagus and they don't know how much more damage. It ate his gums, his tongue. He's on a ventilator right now."
Leo's family said their homeless brother suffers from alcoholism and has been living near the Waffle House for some time.
They added they will push for tougher criminal charges for what happened.
A Waffle House representative said they fired Wilson and another employee who witnessed -- but did not stop the alleged prank.
Police may upgrade charges against Wilson from misdemeanor reckless conduct to more serious felony charges.
A Waffle House restaurant worker in Georgia was arrested on charges of allegedly paying a homeless man $5 to drink a poisonous cocktail of chemical cleaning solution that left him in critical condition, according to police.