Obesity in dogs health risks
Obese dogs are at risk for health problems, the exact same as humans are. Some of the common health problems that dogs get are: osteoarthritis, type 2 diabetes, high blood pressure, heart and respiratory disease, kidney disease, cancer and more. One of the worst things that can occur on unhealthy joints (which a lot of dogs are prone to), is having a large amount of excess weight that your dog now needs to contend with. If you can keep that excess weight off of them, it is a huge help to them.
The ways to combat this are the exact same in dogs as it is in humans. Eating a very healthy diet and getting exercise. Supplementing that exercise with a dog treadmill is a great idea! Switching to as healthy a diet as possible is also very necessary.
Just like us, you are going to want to start off as slow as possible and build your dogs strength and stamina up. I would recommend starting off with as short a walk as possible. I would also guess that if the dog is obese the person is as well. I would recommend getting both of you to exercise and eat right at the same time.