When they book their trip would it be cheaper for them to pay for medical insurance then? Usually travel medical insurance is nominal and I would prefer to pay that then take the chance that I would become ill or injured in the US. Regardless of how healthy someone may seem you can not predict what will happen in the future, thus, insurance.
I have no idea how the car insurance would work, but if you contact your car insurance company they should be able to let you know the cost and then you could determine if it is worth it.