Good Samaritan Ryan Flannery was driving past the victim's home Saturday afternoon in the 1000-block of Infantry Lane when he saw her outside in trouble.
"The scariest part was when he was walking towards me because I think that he thought she was still inside and how did she escape and he wanted to drag her back and finish off the job," Flannery said.
"I asked, 'What happened? What happened?' She started screaming, 'The house is on fire. He tried to killing me,'" Flannery said. "She was like, 'He hit me with a hammer. He tried to light me on fire.'"
Flannery went to his trunk and grabbed a welding hammer and told the man to "stay back."
The woman, Amanda Zelko, remained hospitalized Sunday night and the next 24-48 hours are critical, but her prognosis is good.
Zelko identified her attacker as Timothy Gregory, a relative of her fiancée.
He was arrested at the scene and faces arson and battery charges.
It was immediately unclear what prompted the attack.
Zelko's mother, Linda Zelko, said she is thankful that Flannery was there to help her daughter.
"There is kindness in the world ... people are kind and caring and stopped and took the time to help my daughter in her time of need," Linda Zelko said.
Flannery said: "I hope she recovers and I hope he rots in jail. Nobody should ever go through that."