John Paul Lopez, the caretaker of the home of gunman Ronald Bae, who was shot dead by police during the attack, turned himself in late on Friday.
Twelve people were hurt in the attack in Kawit, 20km (12 miles) from Manila.
Witnesses said Mr Lopez had helped reload the gun. He told police Bae had threatened to kill him.
Mr Lopez was arrested in the town of Imus, close to Kawit.
National police spokesman Generoso Cerbo said officers were investigating the level of Mr Lopez’s involvement but that he “definitely faces criminal charges”, which could be multiple counts of murder and attempted murder.
Cavite provincial deputy police chief Dionicio Borromeo said it appeared Mr Lopez had done nothing to talk Bae out of the rampage.
The dead included a pregnant woman, her young daughter and seven-year-old girl.
Kawit police said Mr Lopez had tested positive for methamphetamine.
Police are investigating reports the pair had been on a four-day drink and drugs binge sparked by problems in Bae’s marriage.
Ronald Bae left Kawit after losing an election for village chief in 2010.