I had similar OP. I had always been tall and thin, but when puberty hit, I stopped growing taller and grew outwards! Boobs, hips and thighs seemed to spring out within months and I got stretch marks in ALL those areas!
My mum, who was always very petite and had never seen a stretch mark, took me to the GP to check what they were! OMG- the utter embarrassment at 13yrs showing the Dr my thighs etc and him explaining it was normal and I'd just grown too quickly! 😩
They are indeed from the skin growing too quickly, stretching and the underlaying structures not being able to grow at the same pace. They do eventually fade to a silvery, wrinkled stripes, but I tried everything to get rid of them. Rose hip oil, early versions of bio oil, and then years later- actual bio oil. Nothing worked and any changes were just the natural skin changes, or me being more accepting of them. I'm mid 40's now and can still see the ones at the top of my thighs and hips- although silvery.
Nowadays, there is laser therapy which can smooth the skin, but I'd guess this is only once she has stopped growing. Silicone gel and strips 'might' help, but I think they are best on raised marks such as keloid scaring, rather than indents like stretch marks.
Sorry this was long OP. I don't know of any treatment that would help if she is still growing and developing more stretch marks. Bio oil is worth a try, even if psychosomatic for her. I do sympathise with your DD though because its a difficult time with this also x