As I said, if I was wrong and you are genuinely concerned about a real person and not with writing fiction, my apologies.
I'm just appalled to think that there are doctors out there who'll carve someone open without any pre-op counseling. WLS is about as serious as it gets, and someone who isn't fully prepared for it--for the supplements they'll be taking, the lifestyle changes they'll be making, and the side effects they might have--it can have dire consequences.
You cannot do any part of your fiancee's journey for her. You can't make another person exercise or eat well unless that person is a young child for whom you are wholly responsible. Your fiancee is an adult. You'll have to decide whether it's worth it to you to marry someone whom you are desperately trying to reshape, physically or emotionally.
Was she 400 pounds when you met her? If so, she hasn't changed; you have. She might be giving you more resistance and arguments because frankly, no one--no matter how uncomfortable she is--wants to be told what to do. You're recasting your relationship into a parent-child dynamic instead of an equal-partners dynamic, and the more you act like a finger-wagging "grown-up," the likelier she is to become a recalcitrant "child."
So get back to an adult relationship. Express to her how concerned you are about her health, safety, and comfort. Ask what you can do to help her follow doctors' orders. Offer to join her on short walks instead of trying to convince her to get on the treadmill by herself (my husband is brilliant at this; if I'm not in a "walky" mood, he'll say something like, "I love that we get a chance to talk with one another when we walk" and I can't possibly say no to that

). Talk with her about why she's had trouble preparing for surgery, but do it without a judgmental, "Why can't you just behave!?" tone.
It won't be easy. It IS frustrating to watch a loved one do crap that's wildly unhealthy, especially when you are so ready to help that person. You're torn between speaking up because you love someone and keeping silent for the same reason--you don't want to hurt feelings.
And it's hard to understand that for someone with a great deal of extra weight, answers are not as simple as "just put down the fork" or "just get up and move more." Read some other threads here, though, and you'll see that for some people, there's a strong physiological component to weight loss that makes it practically impossible unless the person controls what she eats as well as how much. Others are emotional eaters. Others are binge eaters. Still others had traumatic events that triggered great weight gain.
If you've never had a weight issue yourself, you are bound to find it tough to grasp all the conflicting forces surrounding the concept of losing. At my biggest (which you can see on my ticker), getting up and walking around had begun to HURT. The impetus for my weight loss was a trip to a Saints game; I found that walking to our cheap seats in the Superdome was almost impossible for me. I was deeply ashamed of it.
Shame and guilt motivated me to lose--but shame and guilt also kept me from losing for a long time because it was embarrassing even to acknowledge that I HAD a weight issue. I would've died rather than tell my husband that the reason I didn't want to go shopping together wasn't that I was lazy, it was that I disliked how even a simple shopping trip left me winded.
Your fiancee very likely faces the same issue--but she's not going to tell you that. So try to be understanding of what it costs someone who's very large and very unused to physical exertion to get moving. It's way tougher than it looks like, probably way tougher than she lets on.
I've kind of gotten on a tangent, but I'll go back to this: if her weight has been stable since you've known her, she hasn't changed, but you have. Reformat your relationship to be one between equals, not a scolding parent/pouting child one.