This kind of reminds me of how my friendship dynamics were back in the 7th and 8th grades...where "you can't be friends with such and such if you want to be friends with me" kind of thing. Very elementary school-like.

Surely this friend of yours can have her friendship with Sarah and also with you, separately? I hope you or Sarah are not making her feel pressured to choose one over the other, that would put her in an awkward position.