Park Hyun-seo lasts in the sewer until she tries to make a running jump off the sleeping monster up an improvised ladder. While it is momentarily knocked out her father pulls her arm and drags both her and the boy she was protecting out of its mouth, but Hyun-seo is dead.
The family attacks it in many ways, finally succeeding when a bum pours gasoline over the monster and her aunt lights an arrow with her uncle's fire bomb and shoots it in the eye. The little boy coughs. The US was lying about a virus, although we're not sure why.
The experience seems to have straightened her father out a bit and he takes care of the boy in their amazingly resilient snack shack, although always keeping a watchful eye on the river.