I hope everything goes smoothly and I agree with what a lot of others have posted. Be sensitive to her needs, not talk about it a lot, etc.
I've been on both sides of this (although I'd never dream of cutting off a friendship if someone lost a lot of weight!) and I found the easiest situations are where you let the other person (the person who hasn't lost the weight) bring up the conversation of weight. Otherwise just enjoy the time with your friend as much as possible.
I think offering to cook a meal or two is a great one! You can definitely pass it off as helping out and I'm sure she'll appreciate the break. I normally tell people I'm fussy about breakfast (I eat the same thing EVEY single day!

) so I always use that as an excuse to cook for myself too (and others if they'll eat it). I'd also encourage eating out more because you'll have a wider selection of foods so you're more likely to find something healthy (and like mentioned above play the food up as being something you're into right now vs. something you have to eat). for instance, I went out for lunch at work today and they have a dessert buffet. I found myself raving about these cinnamon pears they had there (for only 15 calories each!), it makes me feel better about what I'm eating too.