Even if she wanted such a thing, I wouldn't be party to it. Her life would be shortened for sure at such an enormous weight and I wouldn't want my participation in that tragic outcome to be on my conscience.

Having become immobilized by your fat cannot be a comfortable state of being. Sexually thrilling, yes, but physically hard to live with, I'm sure. Taking care of typical body functions becomes almost impossible - and having all that fat everywhere constantly rubbing up against neighboring fat, along with the inability to sleep on your side or even sit up, certainly makes for a difficult life.
And yet there are real people out there who will do whatever it takes, no matter the expense, the difficulty, or the danger, to appease their sexual appetite for their own enormity.

For me it was a woman nearly 6' tall who insisted she was only 230 pounds but I'm sure was pushing 300.
A lot of the fat was in her ass, though her belly was pretty cushy too. While she was on top her butt was so round and wide, however hard I tried, I couldn't even get my hands to touch the back of it. Thrilling. And holding her hanging belly from that position while doing it was a huge excitement too. What a soft weighty playground she was. In the end a life long fatty, I'm sure.

The average age of a 600 pound woman at death is only 33. Your fiance getting to 1000 pounds will definitely shorten her life substantially.
I love fat on a woman too and the sexual pull to become incredibly obese, or be with someone who is, can be overwhelming - but the dangerous reality of gaining that much weight must be taken into consideration before choosing to steer your body in that direction.

Obviously you can live your life out in response to what your family, friends, and lover think, changing nothing relative to your weight . But will a life like that be good enough for you? Can you truly live happily that way?
If you can answer those questions with a yes, then maybe leaving things alone will work for you.
But if, through serious introspection on the subject, you come to a different conclusion, it will be time to take the heat by challenging your family's & friends' point of view, changing partners, and finding someone who will appreciate the more voluptuous you.
There are, after all, excellent fat admiring resources out there today to help you make the right romantic connection.

I don't think these sexual interests are developed during childhood but instead intrinsic to our being, there in our psyche's from before birth.

I think there are basically places like that around today where you can vacation & stuff your belly to your heart's content. Aren't cruise ships that way, with buffet's pretty much everywhere you turn? And landed resorts where food is the focus must be out there too.
Seems to me if you want to take a pig out, gain as much as possible, button popping vacation, it can be managed.

Give it a try. If it works, and I think it will, you'll both be thrilled with the results.

How about a woman who seems to be hungry every few hours or so and desperate to do something about it no matter how much she's just previously eaten. And when you put her in front of some food, particularly if there's a whole lot of it, she makes it crystal clear there's no place she'd rather be.
Another would be a fat woman who, whenever around food, just can't say no, always eating until way more than full and then, once done, remarking, "I know I shouldn't eat so much but I just can't help myself. I just so much LOVE to eat".

Add sugar and maybe you'll get a kind of cotton candy.