I believe that the ghost they were hunting was the kidnapper- everyone else simply thought that the ghost had to be the girl because of the female forms it took(?). The ghost never was the girl, it was always the kidnapper, as the MC stated the shaman never specified who's bones needed to be exposed.
By opening the coffin and exposing it to light/the sun, he basically exposed it's bones (as was stated before, this is what needs to happen to defeat the ghost) and thus the feast was over with the MC being the sole contributor and victor.
(Or that's what I think I read but my memory can be a bit fuzzy (●´⌓`●))
Can anyone explain how is the opening the coffin of kidnapper end the feast?? Aren't they searching for female body to subdued female ghost? ┗( T﹏T )┛