If you look at more than just the sidewall, you'll notice the tread shoulder is heavily worn. This tire is well beyond 3/32 and verging on bald in most of the tread pattern. There is slight cupping that indicates habitual under inflation by 3-6 pounds. Another factor is that it looks like a lower profile tire (though hard to tell for sure from this angle), which is affected by under inflation even more. There less sidewall support. In turn that causes more flexing and increased heat when under inflated. I bet if you were to ask the wife or the friend, how it felt while corning, they would use the word spongy as the tire slightly rolls side to side under load. They would, if checked, see lots of microcracks about 2" out from the rim on the other tires. They should replace all 4 tires.
Let's look at some other clues. They are in the Desert SouthWest where running bald is done more often because people stupidly think that tread is only for moisture control. I'm betting they have to drive several miles to reach a filling station and that the tires are well hot at that point.
To get a better understanding of the inflation, they should get a dial tire pressure gauge, spend a couple of extra bucks for the one that holds its pressure until released so they can write down the numbers. Then measure all 4 tires right around sunrise. Drive to the filling station and add extra air to each tire based on the cold pressure measured at/before sunrise. They may need to check on multiple days, based on how low the cold tires are. Also I'd be willing to put $5 on that they are slowly loosing air though the hammered sidewalls. Example: cold temp is 29, vehicle placard says 35 for that tire. that tire gets 6 lbs on top of what it reads at the station.
In answer to your other question, I don't think it was driven on long, but the tire is so hammered as a unit that once the sidewall failed it was a like a zipper ripping open all the way around the tire. You can see that by the jagged edges on the tear. There is splaying where the laminate separated but the edges are still crisp and there are thin fibers which would have been worn off if there was any appreciable distance on the rim. I bet the inside is beat up if not partially separatated as well?