If she is driving or walking into Mexico from the US, that is not a problem. Driving back in to the U.S. should also not be a problem. I'm assuming if she flew into the U.S. she already has the proper documents (passport... More
If she is driving or walking into Mexico from the US, that is not a problem. Driving back in to the U.S. should also not be a problem. I'm assuming if she flew into the U.S. she already has the proper documents (passport stamp and I-92 you fill out upon arrival in the USA). Make sure you have those documents with you when you go back. The border has undergone a lot of changes recently, mostly to better expedite the crossing process. Even US citizens are now required to carry a passport to go back in. Any additional permits you may need (if she doesn't have them already) you used to be able to process them at secondary inspection. They have gotten a little stricter about it though, and they may want you to have that already by the time you get to the inspection. If you don't, depending on who you get, they may send you back to get it, then you have to wait in line all over again.
If you are walking, the office you get those documents from, you will actually walk by it to get to inspection point.
I would check with U.S. Customs and Homeland Security to verify documents needed.
