30 minutes of exercise can burn 500 or so calories (depending on type of exercise and intensity). Your average fatty can easily eat that 500 calories in a single snack. Until they get their diet under control there is absolutely no way they will exercise enough to burn what they eat. Additionally, the exercises they do with pisspoor form will cause them injuries, which means they can no longer do the minimal exercise they do. This combined with their lack of diet control means they will probably gain weight and be unable to go back to doing whatever exercise they were doing.