How do I help my daughter to loose weight & understand weight issues-she's13yrs.?

We argue all the time as I feel she eats for the sake of eating, like habit or mindless.I've tried helping by not buying junk food,pop etc, & as she has knee problems it puts her off exercise. She's nearly 13yrs & weighs around 10stone already.She has Xenical but will only take on weekend due to school.How do I help her without hurting her feelings? I've tried being nice,being ugly,shouting,arguing,talking....she has stretch marks & Chronn's disease too,but I really think this is a mental problem.Any advice appreciated,thank u.