I can absolutely see this happening. If he was trying to drive up there to just show her around since it was so hot, maybe something happened with the truck and they were unable to drive it out so they got out to walk or to try to get cell signal by moving around and then things just went down hill.It's a bit difficult to explain, but what I picture in my mind isn't so much John wanting to train Fang.
Because he wrote the texts saying they should start a business together, where they would take Chinese tourists out to the desert and he would teach them survival skills, I picture him wanting to show Fang the desert and show her some of the skills he told her about.
Less about training her, more about wanting to impress her. Completely innocent, should have been harmless fun, but unfortunate circumstances turned it into something else.
I can totally see a man who had served his country in the way he did retiring and then not really finding his place in the civilian world or struggling to do so. Then having some relationship and personal problems with the restraining orders and the divorce and things we've heard about and then having alcohol problems.. then meeting a woman and trying to change things around. I can actually see a scenario where he really was changed, off alcohol, trying to have a fresh start, meets Fang and thinks she's amazing, successful, and just wanting to show her a good time, impress her and then things just went wrong with the record heat and perhaps the truck being stuck or tires going flat and then they can't make calls out for help so they are in big trouble fast.