I have done quite a bit of Google research, but it only applies to minor US citizens crossing the Mexican border, not Mexican citizens doing the same. If this is in the wrong forum, I'm sorry. This seemed to be the best fit for my question. Move the question if you must, but I really need an answer before the end of this Friday (July 31st), as she is set to fly out on August 1st.
Here are the facts: Girl about to cross the border is 16. She is a Mexican citizen, but a permanent legal resident in the USA. She is currently in Mexico and about to travel back to the United States via car.
Her parents are divorced. Her biological father lives in Mexico, and her mother lives in USA as a legal resident. The mother has remarried. The mother's husband / the girl's stepfather plans to drive across the border into Mexico, pick up the girl in question, and drive back to the USA.
QUESTION BASED ON THIS INFORMATION: Assuming that the stepfather is considered to be a legal guardian to the girl, is he allowed to transport the girl from Mexico into the USA without notarized letter(s) signed by both parents?
I want to hope that they can travel across without a problem, but I ask because I don't want her to be stuck at the border when all the girl wants to do is return home to the USA. Any help and source-backed information any of you can provide will be much appreciated. Thank you.

