I think that Elizabeth can be seen as a foil to Abigail in a couple of ways. The first would be that both women are shown to be the primary forces in Proctor's amorous life. The domesticity of Elizabeth is set as a foil to the wild passion of Abigail. Consider Proctor's initial interactions with each. In Act I, Proctor's interaction with Abigail revolved around sexual tension between them, with Abigail suggesting that John was similar to a wild stallion. In Act II, Proctor's interaction with Elizabeth opens with her singing a lullaby to the children in one room, while Proctor seasons the dinner broth on the fire. In both initial settings, Elizabeth and Abigail represent different extremes to the feminine aspect in John's life. In terms of characterization, I would suggest that Elizabeth can be seen as a foil to Abigail. Whereas Abigail centers on power and the desire to control others in the community and gain political control over the difficult situations that arise in Salem, Elizabeth is more concerned with her domestic situation, in particular her marriage to John. When Abigail leaves Salem, taking Parris' money with her, Elizabeth stays by her condemned husband, watching and even encouraging him stand by his principles even though it will bring about his death and her being a widow. These might represent how from a point of view of characterization how Elizabeth can be seen as a foil to Abigail.
In a literary text, a foil usually refers to a character that contrasts with another character in order to emphasize or highlight specific character traits possessed by the other character. Because there is almost nothing but contrast between Elizabeth Proctor and Abigail Williams, and this contrast really throws the qualities of both women into sharper relief, it is certainly possible to consider one a foil for the other. Abigail is ruthless, and Elizabeth is considerate. Abigail is vengeful, and Elizabeth is forgiving.
Perhaps most interesting is the way in which they each feel about John Proctor. Abigail's feelings seem to have more to do with lust: she talks about "how [John] clutched [her] back behind [the] house and sweated like a stallion whenever [she came] near." Abigail also mentions how he helped to disillusion her by making her realize "what a pretense Salem was" and all "the lying lessons [she] was taught by all these Christian women and covenanted men!" It seems like she is most interested in a sexual relationship with him, not really being mature enough to realize that there is more to a relationship. Elizabeth, however, clearly loves John. Despite the fact that he cheated on her, Elizabeth tells him, "I never thought you but a good man, John [...] only somewhat bewildered." Abigail lusts while Elizabeth loves. Abigail is selfish while Elizabeth is generous.