She was not crossing the border. The Border Patrol has checkpoints which have permanent structures on various roads leading out of El Paso. There is such a checkpoint on the westbound I-10 somewhere near Las Cruces, NM, and a checkpoint on the eastbound I-10 near Sierra Blanca, TX.
A few years ago I flew into ELP airport and rented a car to drive up to the races at Ruidoso Downs, NM. I had to stop at a Border Patrol checkpoint near the TX-NM border south of Alamagordo. The uniformed agent looked surprised when I told him I was on my way to the races.
It is a little troubling when you are obligated to answer a policeman’s questions as to where you are going when you have not crossed a border and there is no probable cause for a policeman to have to speak to you. However, I do respect the work of the Border Patrol and the fact that it is extremely thankless.
I’ve driven all over that area in the past and never seen a checkpoint inside the USA except when going into Arizona back in 1957.
Only time I ever saw a checkpoint in New Mexico was after the first weekend of deer season when wardens were checking all cars for illegal deer.