Basically its not as simple as "more exercise means less fat" though in my opinion it might help. The answer is for them to start seeing a nutritionist who can help get to a balanced diet.
My only reference is a documentary "Fed Up" (2014)... well and testimony from everybody who says they gain weight by looking at a doughnut.
As a fitness trainer I think the best thing you can do is not help them lose weight but help them improve their cardio and raise their spirits. There are many things that a good healthy heart can help avoid, and having a good outlook will help them also be more healthy. Generally being more active can help improve lives in many different ways but its not a garrantee to lose weight so I think you should avoid using that as an argument and help them avoid it as well.
I can testify that running for more than 5 hours will result in an appetite that will not go away for a few days no matter how much I eat. That to me is evidence for some form of a biological control that you have no control over and can be thrown off balance by exercise.
I can also testify that a coworker changed his diet with the help of a doctor to start eating many small healthy meals and lost ALOT of weight. I don't know the details but I've heard of this diet from several different places over the last few years. So they should see a doctor or a nutritionist to get a good weight loss result instead of trying to use exercise to do it.
I would like to say fasting from food and water during the daytime may help but only 1-2 times a week. I don't think many people do this though, just another opinion of mine. It puts your body into a fat burning mode where your likely to have ketosis to some degree during the fast.
Hope this helps some. Kudos to you for taking on such a challenge to help other people.