A foil in literature is a character that draws attention to another character by creating contrast between the two.  Amanda and her daughter are two such characters.  Amanda is outgoing, likes to flirt, talk, behaving in a charming, inviting manner. While Laura, her daughter, is quiet, reserved and prefers to remain unnoticed.

Amanda is always onstage, making elaborate speeches about her life in Blue Mountain, describing her youth as a time of popularity, parties and fancy dresses.  It is quite a contrast to the life that Laura lives and has lived.  At 24, Laura has done virtually nothing, except attend the Rubicam's Business College for a few days until she becomes physically sick when they give her a typing test.

In contrast, Amanda is a survivor, even though she is an abandoned wife, she does not give in to depression about her life or her status, what worries her is her daughter and son's potential future.  Especially Laura's future, Amanda sees a life of loneliness and isolation for Laura.  She is also very worried about Laura's financial future.

Amanda has had to struggle all her life, since Mr. Wingfield abandoned her, to survive financially.  She has done odd jobs and probably depended on the generosity of family.  She does not want this for Laura, and Amanda is willing to fight for her daughter's future.

While Laura seems unconcerned about her own future, she is content to sit in the living room and polish and arrange her glass collection, and listen to the old records that her father left behind.

Laura's accepting her quiet life of loneliness and deprivation is in direct contrast to her mother's refusal to surrender to the possibility of her daughter being left out of the joys of life.

There are several possible answers, but two come to mind more clearly:

The broken unicorn and Laura are complete foils for each other due to their fragile nature, the fact that they both exist in a fantasy world, and they are both captive in their own "refuges." When the unicorn broke its horn as it fell to the ground it mirrored Laura's "falling in love with Jim and eventually ending up heart broken". What happened in the end is that both the unicorn and Laura experienced that, once out of their "refuges", they are way to fragile for the real world, and it is likely to destroy them.

The second foil is Tom and Jim: (Jim to Tom)

Both guys share very similar characteristics, but Jim represents is what Tom could have become. Tom is lazy, does not care much about growing up, nor setting a game plan for his life. Jim has made it in life (so to speak), has settled down with a girlfriend and has plans for his future. Both Tom and Jim are haunted by Amanda and Laura (the mom wants Tom to be like Jim and wants Laura to marry Jim)- yet neither Tom nor Jim delivers what Amanda nor Laura want and, in the end, they both end up leaving- each for their own reason.