Don't center the life of your daughter around dieting and losing weight. Don't let her self-image become shattered because of this doctor's pronouncement. Continue to cook healthy foods and serve reasonable portions. She probably won't participate in any physical activity on a regular basis unless she takes some enjoyment from doing it. Can the two of you start taking walks together three times a week to kick off some activity? How about a recreational, non-competitive sports league at the YWCA? Please ask her some open-ended questions about how she would like to handle this part of her life, indicating that you'll support her 100 percent.
It' a lot easier to tackle reshaping your body and exercising if you do it with someone else. As she reshapes her body and makes it more fit and strong, don't have her focus on the scale. Focus on staying active, making gradual progress in the areas of aerobic and anaerobic activity. A consultation with a supportive female nutritionist and exercise specialist might be a good beginning. Don't ever let her fall into the trap of believing her worth is measured by her body size. Focus on the health aspect, not the sex appeal. How you and other family and friends treat food and their bodies exerts a strong influence on her. Good luck. Keep me posted.