Final answer:
The literary device at work when Mrs. Delacroix stones Tessie Hutchinson after being friendly is irony, specifically situational irony, where the outcome is the opposite of what is expected.
Step-by-step explanation:
The literary device in play at the end of the story, where Mrs. Delacroix picks up a stone to participate in the stoning of Tessie Hutchinson after initially appearing friendly towards her, is irony. Irony often involves a contrast or discrepancy between expectations and reality, leading to an outcome that is the opposite of what one would expect. In this case, the friendship turning into outright hostility and violence is a form of situational irony, where the actual outcome is different from what anyone would reasonably expect.