One obvious solution to slim down a fat cat: feed her less. It’s best to do this gradually instead of dramatically. A sudden “starvation diet” can lead to serious health problems. Not only should the cat be fed less but the diet ought to be altered to include less fat and more protein.
And, as with humans, proper diet ought to be coupled with proper exercise. Obese cats, like obese humans, typically don’t exercise, which can be partly blamed on their owners, who may enjoy a placid lap cat but who ought to realize that every cat does require play and exercise.