What kind of car is she driving? What shape is it in? How are the tires? Does she know how to change a tire? Have her take a small gas can with a couple of gallons of gas (if she has a trunk. Take some extra oil, coolant, a small tool kit, pliers, adjustable wrench, screw drivers, folding reflectors, etc. and teach her how to use them.
Does she have a handgun and know how to use it? I assume she'll have a cell phone, but reception can be spotty in remote areas.
Have her plan her route and stops. How many hours or miles a day does she want to drive? Have her join AAA and get a route plan. Have her sign up for some hotel/motel chain plans. Have her take the little guide books from the various chains so she knows where the next place to stay is and whats ahead in the next town. They usually have maps on where to find the motel/hotel. Call ahead for a room reservation.
Tell her to stop early, check in, have a nice dinner, get up early, have a nice breakfast and hit the road early. As a woman, she should stay on one of the upper floors, not a ground floor room.
I don't like to drive more than 8 hours a day. In small towns the good restaurants may close by 8 or 9 PM.
Don't keep valuables visible on the seats when she leaves the car. Have an extra “Hide A Key” under one of the back or front part of the car. Take it from me, you need one of those.
Keep her doors locked. Be sure her cell phone has an in car phone charger, same for an in room charger and maybe her laptop computer. Take her digital camera and email you some pictures as she goes along.
She could also take a small cooler, snacks, water, etc. When I travel by car, I usually take a small electric kettle and French press coffee maker or tea bags and some utensils, spoon, fork, knife, paper towels, etc. My own coffee is better than the stuff in the room.
I buy a small container of milk for the morning coffee or tea to avoid the powdered creamer.
You keep a written description and photos of her, her car, license number and planned route, etc.
Maybe this is overdoing it, but being prepared never hurts.
Tell her to have a good time. Enjoy America. And be careful of course.
I just wanted to thank you for this very thoughtful post!