I go back and forth on what I believe happened (foul play versus innocent wandering off trail), but your comment made go down a different line of thinking.
Let's say Barbara was upset and upset enough to carry that with her through the morning and into the hike. I know when I get upset with my husband I can go for hours in a bad mood and get caught up in my negative thoughts. Maybe she was irritated he wanted to stop and take a picture, so in her already bad mood she decided she wasn't going to wait for him and kept on going. In her negative, irritated thoughts she wasn't fully focused on where she was going and stepped off the trail, only to become helplessly lost in the miserable desert heat. I frequently hike in the desert and can totally see this happening. Several times while hiking in a good mood and paying attention I've gotten turned around in Joshua Tree National Park. It's very easy to do and if you're convinced you're headed in the right direction it's easy to go quite far off the trail before you realize you're lost.
I am originally from a cold weather state, so perhaps my perspective is a little different, but I've lived in southern California for 15 years. I refuse to go hiking in the desert if it's over 70 degrees. The desert ground radiates heat. Combine that with heat generated from movement and the ambient air temperature and it's hot. There is no shade, so the sun constantly beats directly on you. It's not something I've acclimated to. People who've spent their lives living in the desert probably don't agree that 70 degrees is too hot for a hike, but I am always surprised by people who venture out for a desert hike when it's 100 degrees or more. The desert heat is nothing to mess with.