Once upon a time, I was the only non-driver I even knew. Nowadays, I'm often the only driver. I also have a long history of wrongly ignoring things, in the mistaken belief that they don't apply to me ...
... So. My guess is, she may not know that you all have Nexus cards, and if she does, that it's purely because you all drive. She may not be aware that having one non-Nexus person in the car changes the rules for everybody (I'm assuming nobody's made that point directly). I say this because, once again, that's precisely the sort of detail that would zoom way over my own head if I were the one car-free person in the group ... and because she does sound like a very considerate friend otherwise.
If the subject comes up again, it might be good to make sure she knows about the lines, and the huge differences in wait times (which is another factor of which I might not be fully aware, unless I drove that route myself).
Your friend sounds like the sort of person who would appreciate a polite, respectful Word to the Wise. She might have to miss this trip, or catch the bus, but she'd be in better shape to plan future trips with you ... or, if she wishes, to refrain from border-crossing jaunts and stick to more locally-based activities.