Former overweight child here.
Please, please, get this checked out and get her some help. Considering the sensitivity of the issue, probably worth speaking to a medical professional yourself first, and take it from there.
Both me and my best friend gained lots of weight in early teens, aged 10-13.
Both have really struggled all our adult lives to not be obese. She's always been very big and had very serious health issues in her starting in her 20s. My other best friend had similar parents, he had a stroke at 30, wth obesity a big contributing factor.
I went the other way and developed a bit of an obsession with diet and exercise, thus yo-yoing between an adult size 8 and recently being obese. I am now size 10, stable, happiest and healthiest I've ever been, and looking at my childhood with such different eyes.
If she has an interest in a performance sport, allow he to embrace it. Kids are meant to be super active but today's lifestyle doesn't accommodate it, so if she wants to do more sports or exercise at home, please don't curb her enthusiasm.
If she becomes more interested in healthy eating, also let her - provided it doesn't cross over to disordered territory.
If you spare her the 'fat' years in a sensitive way, it'll do her health, mental health, self-worth and adult years a world of good.
Our mums were in total denial, and had such bad food habits themselves, e.g. dessert every dinnertime, kids encouraged to have a 2nd and 3rd helping of birthday cake etc.
Other childrens' mums would regularly raise concerns about our weight gain and the fact that we weren't allowed to do much exercise with our mums, and they totally ignored it and thought we were meant to be 'big-boned'.