This is a fairly interesting question. I think that if we operate under the assumption that Tessie is the protagonist, one could argue that the entire town is the foil to her. The fact that Tessie is the outsider, the individual who is chosen to be persecuted by the townspeople, she would be the foil to the community, who becomes the antagonist in this setting. It is slightly awkward to consider the entire community to be the foil to Tessie. However, in this, one recognizes one of the most basic themes to the short story. The idea of victimization and persecution is a fundamental element of the short story. In this theme, the outsider is punished by the community, who operate as the insiders. Tessie becomes "the outsider" when her name is drawn and the community who take up stones to pulverize her are the "insiders." This would be why the entire community can be considered a foil to Tessie.