Before we get to meet Lacey Pemberton firsthand, we get Margo Roth Spiegelman's opinion of her. And Margo Roth Spiegelman thinks Lacey Pemberton is a terrible friend. (That's the rhino calling the hippo fat.) Why is she a terrible friend? Because she minded her own business and didn't tell Margo Roth Spiegelman that her boyfriend was cheating on her, and she's honest about the fact that she's slimmer than Margo Roth Spiegelman. What a terrible human being.
So Margo, being a perfect human being in her own mind, vandalizes Lacey's car by smashing an entire catfish under her car seat. Gross.
But since Lacey Pemberton is actually a nice person, she worries about Margo when she goes missing. She worries that Margo is dead, and she tags along for the road trip to find her.
What a mistake. Margo Roth Spiegelman (more like Margo Roth Spiegelmean) is cruel to her as soon as she sees her again. She makes fun of Lacey for dating Ben and still can't drop it that Lacey made her feel fat. Lacey returns to the hotel with Ben, and we never get to see if she still wants to be friends with Margo. But we doubt it, as her last words to Margo Roth Spiegelman are "It's been a real pleasure knowing you" (3.22.34). Note the sarcasm.