“They are only good for 6 months”
No IDP are good for 365 days I keep one for world travels up to date ever 365 days.
You can rent a vehicle with a IDP and your foreign DL, you also can state register a vehicle using a LLC or S Corp incorporated either here with foreign ownership or incorporated in a foreign country. I have two vehicles registered and plated in WY to a LLC out of Wyoming because Texas won’t title or plate then but Texas must recognize WY plates due to reciprocity. I am about to go to Mexico and get a BYD register it in that same LLC ,plate it in Mexico and drive it across the border it’s a corporate vehicle then ,and Texas cannot via NAFTA say it can’t be here.
There are ways for her to have a vehicle registered, she could have had Mexican plates as well as Mexico will issue its citizens driver’s license and plates even if they are here Mexico enables expats in every way they can.
When I moved back I never bothered to get one. I had a US license and a foreign license. I always kept my foreign one for when I was driving someone else’s car and got pulled over.
“Mexico will issue its citizens driver’s license and plates even if they are here”
REALLY?