Wow, this hits close to home. Bill is a local guy from Marietta, my Atlanta burb. It appears he's quite experienced, and hikes annually in Joshua Tree.
Here's a video link with a little more detail. He was only equipped for a dayhike, which is not the best possible news after almost a week missing. Keeping my fingers crossed for him.
Death Valley and Joshua Tree are my two favorite winter hiking destinations, but I've never done either one in summer. I've been to both several times in summer, but no hiking - it's just too brutally hot to be enjoyable.
Western trips have not been kind to Atlanta area hikers recently. A man from Dunwoody died in a fall on a trail below the North Rim of the Grand Canyon late last year. I'll be in the Sierra in less than a week, and I'm planning on being just a tad extra cautious . . .