I disagree. With tons of respect, of course.
First off, overweight cats are much more prone to diabetes and heart disease and other diseases later in life, so it is very important to get the weight off. That being said, you want them to lose it very slowly. Try googling "Hepatic Lipidosis". I'll give you a quick run down of the disease, but you should still look it up because I'm not the best at explaining it.
When overweight cats have their caloric intake cut by too much too fast, and they start to break down all that fat, it gets all clogged up being metabolized by the liver, and the liver shuts down. And they die. Within hours, usually.
Anyway, feeding your cat ad lib (free choice) is not the way to go. Even if you are only giving her a certain amount in the morning that has to last her all day, she might eat it all in one sitting, consume no more calories for almost 24 hours, and go into hepatic lipidosis. So you have to divide it between two or three meals in one day.
Also, many quality cat food companies make a light version of the food. I feed my chubby cat Royal Canin Slim Cat. He has went from about 18 pounds to 14 in a year and a half, and my boss says this is a great rate for him to lose the weight.
And I have 3 other cats and a dog, so they have gotten used to what time is feeding time, and who eats out of which bowl. I still stand with them the whole time they eat, but they have learned since I stopped feeding them ad lib. Like, the dog gets dog food, Sampsonite my big fat baby gets his Slim Cat, Mika and Etna get Royal Canin Adult, and Ava gets Royal Canin Baby Cat, because that's what she is! hahaha!
Anyway, good luck with this, and I'm glad that you realize an overweight cat is not a healthy cat. We have clients at work that, you know, we tell them they need to put their cat on a diet or it's going to have problems later in life, and they think it's just hilarious that they have such a fat cat. It's just sad.