I think part of the problem is just that we're not the target audience. The book seems pretty squarely aimed at tomboyish girls, who might very well share Astrid's "Blech!" reaction to Nicole's interests and might identify quite passionately with Astrid's feelings of rejection, so maybe Jamieson felt like she had to make Nicole a pretty stellar friend so readers wouldn't flat-out hate her.

I also think maybe I've understated Nicole's side of the feud? I think she's less to blame than Astrid, but she's not totally blameless.

But I really do want them to continue being friends. But they can have more than one friend! They can both have friends for their ballet/roller derby interests, and continue being friends with each other!