My fiancee is fat and wants to reduce her size and weight, especially her belly. What should she do?

Diet and exercise. For women, 1500 cal/day balanced between carbs/protein/fats with 30 minutes of cardio exercise 5 times per week should get her healthy. It may take 6-9 months to see full effect. Sometimes we use medicines like metformin or spironolactone in women to decrease their androgens, which can cause belly fat. I'd recommend a visit with her doctor as well to rule out any metabolic issues.