I can imagine she was at a gas station - according to the poster cited above, she had gotten as far as where Highway 185 turns off to the major town of Lagunas, makes sense it would be a location for a service station though I can't tell from google earth, only that there is some kind of building there.
I can also imagine someone eg the attendant (in Mexico stations still have attendants pump gas) and/or maybe her ride who wasn't going any farther, was maybe helping her find a ride or just paying attention as she approached vehicles to ask for a ride. She would be very conspicuous and memorable and the clerk etc would all be watching her. The truck driver would be sharing the info that he was heading back to take a roundabout route, since he had been blocked by a strike, certainly information that would be very interesting to anyone in a gas station or heading in that direction. The attendant or whomever, might have commented on the situation of this tourist getting in the truck to the other staff, so they would all confirm it weeks later.