The woman, a mother aged 20 named as Kepari Leniata, was stripped, tied up and doused in petrol by the boy's relatives in Mount Hagen in the Western Highlands, said the National newspaper.
She was then thrown onto a fire in front of hundreds of people.
Police and firefighters were unable to intervene, the paper said.
The Post Courier newspaper said they had been outnumbered by the crowd and chased away. Both newspapers published graphic photos of the incident on their front pages.
Provincial police commander Supt Kaiglo Ambane told the National that police were treating the case as murder and would arrest those responsible.
In parts of the Pacific nation deaths and mysterious illnesses are sometimes blamed on suspected sorcerers. Several reports have emerged in recent years of accused people, usually women, being killed.