Michael Brandon Jemison turned himself in to authorities on Wednesday -- two days after the incident. He is charged with the murder of Justin Jones.
Jemison's girlfriend was arrested on charges of failing to report a felony.
Surveillance video from Walmart showed Jemison and Jones arguing with each other, Tomball police said. According to investigators, Jemison hit Jones with the helmet, causing him to collapse inside the store.
Jones died at the hospital.
His girlfriend, Lindsey Stover, told ABC13 that Jones was withdrawing money to give a friend before the encounter with Jemison. She believes it was only about $60.
"I don't think it was spur of the moment. This guy saw an old enemy," Stover said.