How do you motivate a morbidly obese person to work out?

First, educate her on nutrition, it's benefits and how weight loss can actually reduce/eliminate some diseases like diabetes and hypertension. Then, start the exercise process slow. Use a site like mypyramid.gov to determine how many calories she eats/burns on a normal day. It can also aid her in how many cals she should eat to drop a safe amount of weight each week. Let her pick healthy foods she likes to plan her calorie intake, while starting exercise at something simple like chair dancing, and slowly increase exercise each week. Walking 2 blocks might be too much for her. Get her to walk to the mailbox once or twice a day at first and increase the walking as she feels better.