I feel like this is worth discussing even if the op is fake.

There are a lot of factors, but one of them is also how desperately you want to remain mobile, and what you consider immobile. For instance, vanillahippo is over 800 lbs, but also 6'1 or 2 and literally big boned. She's effectively house bound, but can stand and walk on her own and get around her apartment and use the bathroom (I don't know what her set up is but it's not a bedpan.)

Echo is comically short, so she looks even more massive than vanillahippo while weighing 100lbs less, but because most of the weight is in her legs and ass, she can still stand and ever so slowly shuffle to a toilet. Technically she's not immobile, but she's almost always in bed now and getting to the bathroom is exhausting...so she's right on the line, to the point that "mobile" wouldn't be accurate.

Jesstastic has the body of a Barbie doll with the gut of a 450 lb pregnant woman. She's already suffering aches and pains and 500 lbs will probably keep her in bed.

All that being said, unless you suddenly suffer an injury or an extreme illness, immobility isn't going to sneak up on you if you're trying to stay mobile. You'll know when it's getting close and it will be a long time coming. The only way you'll be surprised is if you give up. That's how it happens. Either because of depression or pain or enabling, you just decide to stay in bed one day, and someone caring for you makes it possible for you to stay there. And a day turns into weeks. Months. One day you decide you want to get up...but your legs are too week and you fall. So you rest, before giving it another try...and never do.

Although if an unuussual amount of fat goes to her bum and legs, that could be a sign of lipedema, where her legs will eventually fill with fluid, looking like balloons ready to pop. So here's hoping

Then you MIGHT be a death feedist

First, lay down two safe words right away. One for slow down/take it easy, one for full stop. A lot of the struggle of a dom getting into being a dom for the first time is worrying that they're going too far, when they're partner wants to get roughed up or degraded or whatever even more. Having to keep asking if someone is ok can ruin the mood.

Watch some bdsm porn, make a list of things you want to try, and talk about it well before hand, that way when you're ready to go you can just go at it. Then talk about it after when you're done.

Also, have your partner read up on how to perform a stage slap to the face. A good loud slap can blow your tits off, but an actually aggressive and poorly aimed slap can cause real damage.

As long as you make clear what you're doing, what you're planning to do, or your partner has explicitly expressed that they are ok with what is happening to them and have no desire to change, whatever's clever, man.

While some get off on the moment of death by fat, usually by heart attack, I'd wager that for most, actually dying from or killing someone with their fat isn't the GOAL. The turn on is to get as fat and as fast as possible. That someone can't help/stop themselves even while knowing how quickly it will end their life enhances this arousal. Fear, danger, helplessness, looming death...these are sources of adrenaline for someone who is trying to move as little as possible. Every calorie beyond what your body requires is inching you closer to the edge of a cliff. For some, it's worth knowing they are actually going to fall so they can live with that high until they do. Conversely, I imagine many death feeders don't WANT their partner to die, but get off on the fact they their piggy can't stop no matter what it does to their health. Watching someone continuously grow without stopping until it kills them is enjoyable right up until the end. You want them to get as big as possible and be with you as long as possible, but you won't stop feeding them until their heart explodes.

For me it's not so much the specific health effects, but that you're partner knows they're killing themselves, but they are too far gone to stop it from happening. The only difference between fantasy vs reality is that in reality it's because they have a similar fetish that renders them helpless, and in fantasy and literature it's purely or a binging disorder or a stomach stretched out of emotional dependence. The idea that unless you help them there is no turning back, no hope of longevity, and that they don't realize this until they are completely dependent is incredibly hot.

Your death will be delicious

You're going to die young, from fat. You're going to gain as much as you can as fast as you can without stopping until your body gives up and your heart explodes. How long is enough, Knowing this is your end? 20 years? 15? 10? Just a few with the right sick and rich fucker to help you along?

There was a time when they had a choice, but it's gone now.

There's nothing quite like watching a piggy reach the point where they are so addicted that they can't help themselves from eating to the point where they are so full that the pain overcomes the pleasure every time. They have to eat everything in front of them until it hurts so much that they can't enjoy themselves for a couple of hours. In that time they reach a moment of clarity and realize they want to stop, wish they could stop, fear for the future...but as soon as they digest and the pain subsides...only chewing, swallowing, and cumming matters.

When your piggy is physically capable of escaping their fate, they have no will to stop. When they briefly gain that will, they are physically trapped.

Only you can save them, but you won't.