Darlene was worried about Ben finding out about David, and David finding out about Ben but she sends both of them a text-message about her flat tire. Then, after David arrives first, she shows shock and worry when Ben arrives.
When Darlene reads her text to Ben, she mentions that she's stuck on "the" I-90. Saying "the" in front of a highway name is a colloquial expression used by people from Southern California (where the show is filmed). People from Illinois would simply say they were "on I-90."