All dogs need is adequate exercise and a healthy diet. If you can not provide these simple things for your dog than you shouldn't have got one in the first place.
I can understand that people like to treat there dogs, e.g. giving the left overs of the Sunday lunch, however, can the owner stand to inject their dog every day when it has diabetes, or to have people look and comment on the size of the dog. I also believe that if your dog is overweight, it is animal cruelty. I can understand if your dog has a medical problem, but apart from this it is not acceptable. If your dog needs drugs to save its life, which to be honest this is what an overweight dog is at risk from, then it should be treated with drugs, however, the dog should not be given back to the owner, unless they go onto a course to learn about the needs of the dog and what damage is being done if the dog is overweight!