Maybe I'm in the wrong here, but to me, the whole "It's my body I can do what I want with it!" ends the moment you enter into a serious intimate relationship. Yes, it's your body and you have to live in it, but it's also the body that your partner has to look at naked and touch and fuck. You shouldn't go get serious permanent modifications or anything that risks your health of course, that applies to anything and everything, but if it's something like hair, clothing, and basic self care like hygiene and not being a lipid blob, then it just seems so selfish to me not to make an effort for the one you claim to love.
I know of a couple of marriages/LTRs recently that are now over not entirely due to a weight issue, but weight being one of the many factors causing long-term unhappiness.
In one case it was the male partner's constant gain, resulting ill-health, his loss of interest in any social life, and his stubborn, bad attitude about it. His refusal to do anything about it and also trying to gaslight the slim, fit, far more attractive female partner about her looks to reassert some power caused massive resentment, anger and loss of attraction of her part. Add to the fact she had far more atractive male prospects knocking down her door, so to speak, and it wasn't hard to see where this would eventually go. In the other case, the woman's piling on weight within a short few years on top of other major domestic issues regarding child-raising led to the male's loss of attraction, which he admitted, after which it was fire and fury on her part, and game on for divorce.
It IS your body, but in a long-term relationship based at least in part of physical attraction, the state of it does matter, esoecially when it is absolutely within your control should you choose to exercise it. It's a really difficult issue to negotiate without stepping on feelings, which is why so many people put up with it for ages without saying anything at all. The dam eventually breaks when the fitter partner has someone else attracted to them and realises they are actually experiencing physical attraction again.
The woman in the story sounds like a psychological mess to start with. Suicidal ideation, depression .. probably the weight wasn't the ultimate deciding factor for this dude to split (esp. given he was attracted to her from the start at the same sze) but living with a perma-miserable, stressed out person with major psych issues on top of morbid obesity isn't exactly fun, I imagine, and dude may have woken up to realise he didn't want to look down the barrel of many more years of this lifestyle. The fact he started to lose weight indicates he may have had another, less stresssful prospect on the horizon. He may also have simply realized the number morbid obesity does on health and ability to get out snd do stuff, and simply wanted a different life.