Does she belong to any organization that offer car rental discounts? AAA? Her insurance company? Credit union? If so, she might get a group discount but she is still under 25 and she will pay the age penalty.
I’m 22 and have rented from Enterprise. She must have a credit card in her name, and pay a $15 surcharge.
It must be an actual credit card, and not a debit card.
This may not be helpful if she needs the car to drive a significant distance from the airport to where you live, but if not you could rent a car for yourself and let her drive your car.
IIRC, when I was younger and I rented a car through some affiliate of my auto insurer, there wasn’t any extra cost for young drivers. Plus, the “supplemental” insurance charged by the rental company was automatically covered by the insurer.
It’s actually turned out that some of the discount agencies have no surcharge or restrictions for her. Should be no problem. Other than the usual rental traps and gouges…