There is a lot of medical, social science, and anthropological evidence that peer and societal influences on an individual's health habits influence their behaviors and outcomes. This is the case for smoking, drug use, but most of all, diet and exercise. As children go forth in school and begin to socialize more in peer groups, their peers start to influence their fitness and nutritional habits. A child with friends who are sedentary and have bad nutritional habits is more likely to have these problems as well.
Ultimately, it is important for parents to try to exert an opposite influence on their overweight children's diet and exercise habits. While you want to avoid alienating her from her friends, you should educate her about what the best types of foods are, what foods to avoid, portion sizes, avoiding sugary beverages, exercising regularly, etc. Your daughter's pediatrician
can help with this, and may ultimately wish to make a referral to a nutritionist