They were so fat they literally wrapped around her.
I’m not petite, but I manage to stay within my seat’s area, with armrests down. Oh, and before you ask, no, I will not trade seats with you. In fact, I won’t even take my headphones off to entertain your request.
At least one of the pics appears to have been taken in “selfie” mode and result in a mirror-imaged pic. If you scroll down to the pic of the tray table, you can read “LIFT” on the seat belt, so that image is not mirrored - meaning the purple-shirted passenger is definitely sitting on her left. Which would mean the white-shirted passenger is sitting on her right. That means the pic of the purple-shirted passenger’s shoulder is mirror-imaged.
The shoulder shots don’t indicate a problem, but the tray table picture does. Given her description of the circumstances - she couldn’t use the armrests, and she couldn’t properly use her tray table - yes, she got shortchanged, and I think it’s fair for her to be upset, and it’s fair for her to expect some compensation from the airline. Reasonable folks can disagree over what should be considered “fair” compensation for her subpar customer experience; she shouldn’t expect free passage, but I don’t think the airline should have flatly refused to show her any consideration.
How would requiring a large person to buy an extra seat work? Would the airline web page make that an option when making the purchase… “Would you like to buy the seat next to yours?” Then it would be an option for everyone. I cannot see the airline asking a passenger if they are large. If the person declines to buy another seat up front, and the flight is full, as usual, there is no way to even offer the extra seat purchase real-time.
The armrests, while delineating space, do not work well for me - too low usually. I have to slouch if I want to rest my arms, which causes knee issues with the seat in front of me. Also, the shoulders, as mentioned.
I always choose a window seat, so the armrest to my right is mine and mine alone! LOL
Huh. Is that a thing with Android phones? Photos are reversed? Weird, selfies on the iPhone look completely normal and are not mirror-images.
And …why? Because they think people are used to seeing themselves in a mirror, and it doesn’t look right if it isn’t mirrored?
My Android phone has a setting of “Save selfies as previewed” Save Selfies and selfie videos as they appear in the preview without flipping them.
This is a setting that has to be turned on to avoid flipping. I don’t remember what the default was or if I changed it any point.
Yeah, pretty much I think.
You know they have window seats on both sides of the plane? 
You just made me realize something! I always choose a window seat on right side of the plane. Always! Why? I’m not sure because I just now realized that I have been unconsciously doing that all along, but I think it’s because, in bed, I mostly sleep on my right side. On a plane, I turn my body towards the right so my head slumps towards the window.
My hourly rate is $500. I’ll PM you my venmo info for payment. You are welcome! 
I seem to have a vague recollection hearing about an airline that does exactly that. It isn’t just for “passengers of size” (although maybe that’s part of it). It’s for anyone who wants some extra width without needing to pay for a full first class seat and all the other amenities that come with it.
Or maybe I was thinking of Air New Zealand’s “Skycouch”, where you can book a block of three economy seats on along haul flight, giving you enough space to lie down in a fetal position and sleep during the flight.
Do you take Blue Cross & Blue Shield PPO? 
One way to be fair is that each person gets 1 and 1/3rd of an armrest. The people in the outer seats have a full armrest plus they get 1/3rd of the middle one. The person in the middle gets 2/3rds of each armrest. This would mean each person gets the same amount of armrest real estate.
But probably the solution where the overall happiness of all passengers is maximized is when the middle person gets both armrests. That would alleviate the discomfort of being in the middle. The middle person is happier since they get both armrests. The outer people have the more comfortable seats, which balances the loss of the use of the middle armrest.
I’ve read the story and seen the pictures.
The person had an absolute right to complain. The two people either side of her were too fat to fit in a single seat and they should have either been moved by the airline or removed from the plane.
Shame you are fat but it when you spill over into another person’s seat to the point where they can no longer use it in a reasonable manner then it is up to you or the airline to rectify it.
This is the etiquette I’ve heard from multiple sources. The middle person doesn’t have the window view or easy aisle access, so let 'em have both armrests.
I do not disagree w your sentiment. I’m just reporting the facts as they are.
DoT’s remit as applied to the airlines is purely consumer protection. All matters of safety are delegated absolutely to FAA. Which is, admittedly, a division of the DoT, but one with a lot of independence dating back to its days as an independent free-standing federal agency.
And yes, the whole “Do we need bigger seats and less-packed airplanes (and airports)” is a fraught political problem that both agencies are trying like heck to avoid dealing with and so are using every bureaucratic excuse (both good and bad) they can muster. IMO as a seasoned observer of these two groups, if anything ever happens on this front it will start from an unequivocal high priority demand from Congress.
Ultimately, evacuation is part of aircraft design standards required for certification. And is subject to certain required testing procedures on new designs. The design & corresponding testing standards date from the 1950s and effectively require the testing to be done with able-bodied adult people who all can read, speak, and hear English, and who all have normal mental capacity, vision, & hearing. None of whom have a child or dog in tow, much less 2 or 3 of each. And none of whom will grab their purse, laptop bag, or overhead roll-a-board suitcase before starting to move towards an exit.
We can certainly argue that these are unrealistic testing standards. We can also argue about whether the standards on number and size of exits is too many, too few, or just right. And whether the required elapsed time for a passing test is too slow, too fast, or just right.
[aside]
I just lost 30 minutes to a fire alarm evacuation of the high rise I now live in. It appears to have been an alarm triggered by excessive construction dust from work in a corridor.
[/aside]
I’m not sure I have a punch-line to this whole discussion. We collectively are in a not-so-good spot, with no easy nor low-cost ways to improve matters. But I will say this:
IMO we should not use the battle cry “Safety!” as a bludgeon for consumer protection. Nor should we use consumer protection / convenience as a tool to circumvent safety. Let’s all at least try to be intellectually honest as we design our society. That’s a variant on the idea of everyone being entitled to their own opinion, but not to their own set of facts.
I think it’s pretty safe to say that even paying to book two adjacent seats, many airlines will undoubtedly overbook and then ask you to give up that second seat… Hey, if you put up a fuss they’ll just kick you off and then the airline gets paid twice for two seats rather than just double-dipping on one.
AIUI, the general idea of a large passenger buying two seats is to avoid them intruding on the personal space of adjacent passengers. So if the airline overbooks and tries to put another passenger next to the large passenger, then it’s that other passenger who gets shortchanged. OTOH, if that other passenger’s alternative is to be left behind, maybe being wedged in next to a large passenger is the lesser of two inconveniences.
Also, if the large passenger has paid for that second seat, the airline doesn’t gain anything financially by having another living, breathing passenger sitting there. In fact, they will suffer financially because they will have to comp the large passenger for taking one of their seats back (not just the fare that was paid, but actual overbooking compensation).
At the same time, the width of seats on many U.S. airlines has shrunk from about 18.5 inches to 17 inches. And seat pitch — the distance from one point in a seat to the same point in a seat in front or behind it — has decreased from an average of 35 inches to 31 inches. On some airlines, that distance is 28 inches.
Wikipedia sez:
In 1985 none of the main four US carriers offered a seat less than 19 inches wide. Since the beginning of the 21st Century until 2018 average seat width decreased from 18.5 to 17 inches, and sometimes as low as 16.1 inches.