The name of the woman standing outside Go's house is Andie. And she's Nick's mistress. Wait… What? Did he just say mistress?
He did. She's young, she's pretty, her name is Andie, and they've been together for a while. Nick says that now that we know, we'll stop liking him.
Andie the Mistress is freaking out because she saw the press conference on television, Amy is missing, and Nick hasn't been taking her calls. This means that she's the mystery caller on the disposable phone. Then she invites herself into Go's house and starts taking off her clothes. Amy is missing, and that stinks and all, but what she really wants is sex. Now.
Just in case you're still reeling from the news that Nick is a nasty cheater, let's back up and find out just how this happened. Andie was Nick's student in a class he taught called How to Launch a Magazine Career. It was his intention to use the class as a platform to talk the all-female class out of going into the profession, but having the attention of fourteen women was something he just couldn't resist. Therefore, he rewrote history so that he's still a mega-successful magazine writer who's temporarily relocated to North Carthage to help with a family situation (which, according to Amy's diary, he isn't doing much to help with at all).
Meanwhile, back at home, things with Amy weren't looking good. She was distant and depressive and unresponsive to his needs—so Nick began to fantasize about Andie as an escape.
Finally, during a freak April snowstorm last year (about the time Amy was writing the diary entry we just read), Andie came into The Bar. Nick made her some drinks and they started flirting, and one thing led to another. Before he knew it, the Andie Affair had begun.
Nick initially felt like a creep for cheating, but eventually rationalized that since Amy isn't doing her job of making him feel like a man, someone else had to do it. He'd wanted to ask Amy for a divorce, but her doting parents and the fact that she practically financed The Bar have kept him from fessing up.
Now Andie's clinging to Nick and trying to convince him to make whoopee in his sister's house while his wife is out there somewhere missing or dead. Nick, though, can't stop thinking about all the ways their affair could have been caught on security cameras outside Andie's apartment building, maybe even by police surveillance outside Go's. Even though he bought a disposable phone specifically for their calls, all his voice mails and calls to Andie's cell phone could still be traced.
Nick persuades Andie not to tell anyone anything about him, and that eventually, after the Amy disappearance comes to some kind of conclusion, they can be together. Andie agrees, but gets Nick to also agree that she'll get to talk to him every day, even just for a few minutes. They fall asleep on Go's couch.
Nick wakes up at 5:00AM and freaks out because his young mistress is sleeping with him on his sister's couch. He forces her out the door and she reminds him one more time to call her every day. Just as she leaves, Nick turns around to see Go standing there, having seen the whole thing.