Here is Randy's reply. He is all knowing of things Death Valley and has a web site and blog. His info should be helpful to you.
Hi Terry,
Sorry I was away from the office yesterday and couldn't reply.
Although one could reach the Riggs site in 2WD, it would have to be something with high clearance. If he is going to rent a vehicle, I recommend it be an SUV. The biggest danger in driving would be a flat tire, but slow and easy would do it with reasonably good street or all terrain tires.
So, it is not the 4WD that is necessary, but the higher clearance that comes with a 4WD. I can't imagine any sedan having the necessary clearance.
I can tell him that the readers of my Death Valley Talk forum on DeathValley.com would love to read this story below. If he would repost this there, he may very well get a response from somebody willing to drive him out there.
Here is the link to the forum:
http://www.deathvalley.com/dvtalk/dvtalk.shtml
There are other discussion board for Death Valley that he could find via Google.
I hope this helps! Thanks for your email.
Best,
Randy