Well, for starters, how about educating her about eating right.
Not the atkins or any other fad diet, but just plain old, eating healthy vegies, fruits, and that sorta stuff. Hey, maybe you can get her on a Raw diet?
Second, what about getting her a gym membership and you both can go in the evenings. If not get a personal trainer, they're really good but a bit expensive.
If no personal trainer, then you can spend time working out with her in the evening and guiding her. Even if it's not a gym, going on brisk walks or jogs together early in the morning or in the evenings would be a great start.
Honestly, if these ideas are too boring and depressing, then start a fun activity - sports or dancing are great activities to lose weight and develop at something.
and most, most important: first get her to accept the problem, set goals, set a method / plan and a deadline/timeline. having that clarity alone will do wonders. Let her know that she isn't "fat", period - she is just a certain months away from being fit.
Its an awesome awesome bonus that you still see her as the most beautiful woman on earth, and use that to reinforce her self esteem when it's really hit rock bottom.
But remind her that she needs to feel good about herself too - she needs to feel she is attractive, and not just you.
good luck!