Don't think I've run across that yet, but I have told my neices that they need to size up because they don't wear their clothes right. Big mistake ... that's the way everyone wears them! Okay ... I prefer to hide my muffin roll, but they want to show it in order to get their pants tight enough? Would work with long tops, but they wear crop tops.
You can't "tell" anyone how to wear their clothes, but you could purchase her an adorable outfit (in the other size) and see if she "feels" better in it. Sometimes it works and sometimes it doesn't, but ...
Fate whispers to her, "You cannot withstand the storm." She whispers back, "I am the storm."