How does someone with severe arthritis of the knee lose weight?

Low impact exercises. Try gentle, low impact exercises, such as walking, swimming, water aerobics, step aerobics, yoga, tai chi, stretching or chair exercise that is done while sitting. You can burn some calories and increase flexibility by doing any of these regularly. Eat healthy and don't overeat.
Try swimming! Exercise will help with arthritis, which will generally reduce the levels of joint pain one has. Swimming and aqua therapy are great forms of exercise for someone with arthritis.