Lots to choose from rental places these days. They would pretty much need to do some homework and shop around online or you can do it for them if they don't have access to a computer. When we went to Yellowstone, some years back, we went through Avis Car Rental Co. In the long run, it was better to put the miles on a rental than our personal car.
At the time they offered "Unlimited Millage" for whatever kind of car we rented. Which I thought was a great deal, so all I had to do was purchase gas for the trip along the way. The other thing is that they tried to get us to purchase their car insurance, and they will put a lot of pressure on them to buy. All rental companies do as it's a great money maker for them. I would suggest to them NOT to purchase this, even though rental places will make you think its really needed. But this will be up to them and their choice if they decided to want it.
Most car insurance companies will insure the rental under their own policy for the entire time they are renting it. They do need to check with their local insurance company to make sure though. They will also need to make sure that they document every little scratch and ding on the rental, (inside and out) as they will charge you for the repair when it's returned and not stated on the original papers when renting.
The other thing that they did was when we showed up to pick up our rental, they didn't have the one we pick out, so they upgraded us to a better rental for the same price as the one we wanted originally. Some companies will offer a free upgrade as part of their specials online or otherwise advertised.
So they need to shop around some and see who is offering the best deal for the money. The sooner the better as they can reserve a rental for whenever they are ready to leave. As the months get closer to summer, sometimes rates tend to go up some because their getting into the busy season for driving and people taking vacations and other trips.