Noelle may call herself Amy's best friend, but she's really one of the most important pawns in the master plan to undo Nick. Noelle isn't particularly smart, which is precisely why Amy chooses her. As she puts it, she's "nice enough, but with a soul made of plastic—easy to mold, easy to wipe down" (36.4). The Pottery-Barn-compilation-CD-loving, coffee-table-book-collecting Noelle is the perfect person for Amy to emotionally control.
Oh—and she's also pregnant, which enables Amy to steal her pee in an elaborate scheme involving a whole lot of lemonade and draining her toilet.
The plan definitely works. Noelle is a ticking time bomb, and eventually the growing suspicion against Nick causes her to explode. In public. At Amy's candlelight vigil, during a moment of silence after Nick's speech. She not only accuses Nick of murder, but also announces Amy's pregnancy to the onlookers, the media, and the world. It's mission (easily) accomplished as far as Amy's concerned. Noelle may not be the brightest crayon in the box, but she sure knows how to create a plot twist. Or not—she's only doing what Amy's set her up to do, after all.