Abby, Carol's BFF, tends to pop onto the scene out of nowhere at the most inconvenient times for Therese. We say inconvenient because Abby is even more patronizing than Carol, if that's possible. She says things to Therese like, "Are you old enough to smoke?" (7.163). Thanks for asking, Mom.
Maybe this is why Abby and Carol didn't work out romantically—they're too similar. One of the reasons Abby is so demeaning toward Therese is that she's jealous Therese gets to be with Carol and she doesn't.
However, Abby is still a good friend to Carol. She stops the jealous ex act fairly quickly because she wants Carol to be happy—she really does care about Carol, and she's willing to put Carol's needs above her own. She cares about Therese, too, in her own way, saying, "I don't want you to get hurt. I don't want you to hurt Carol either" (10.77). Abby, you might say, is Therese and Carol's only cheerleader.
Abby probably could have shoehorned herself into the road trip if she were the meddling type—Carol likely would have let her—but instead Abby sends along a stuffed monkey, Jacopo, as her proxy. It's like a roaming gnome, allowing Abby to be on the trip in spirit.