A person could, technically, lose a lot of weight and still maintain large amount of food consumption. I have seen it done.
If a person eats whatever they want and there body is at stasis -- not losing or gaining any weight -- and they do no extra cardio or weight training, then by adding intense cardio -- 20 minutes of sprint/jog intervals can burn a lot of fat -- and weight training they can lose weight rather quickly at first.
However, after they lose a significant amount of weight, then their body will no longer be able to stay at stasis eating the amounts that they did before and the weight may come back on or they will stop losing weight.
The key to constant weight loss is pushing your body to the limit and keeping that intensity up. If you do that, then you can continue losing the fat by making small changes to their diet.
Now with that said, it is always best to add a good diet with an exercise program for best results.
One more point, weight training does burn a lot of calories during the session and after, but it also helps to maintain muscle mass which keeps the metabolism high. Without weight training the body will, unfortunately, use muscle -- along with fat -- as a fuel source. The body will not store what it does not need. Without some form of resistance training, then your body will have no use for the muscle it has accumulated from carrying the extra weight for all those years.