I completely sympathise. I went from a tall, thin 12yr old then stopped growing up, and grew outwards! Boobs and thighs from nowhere! I got stretch marks on my breasts, thighs and buttocks at 13. The worst part, was that my thin mum took to the GP as she had no clue what they were 
Over the years, I have tried ALL sorts. Bio-oil, creams, rosehip oil and absolutely nothing made any difference. They will change from an angry, purple colour and eventually go silvery which is less noticable.
There is a silicone formula which has been proven to help scars and stretch marks though. It comes in a gel or patches and needs to be used for months to be of any use. (I've never tried it myself though. My marks are so extensive, that I'd spend a fortune on it, plus, I'm in my 40's now and not so bothered.)
One brand is dermatix. Its expensive, but with the gel, you only use a tiny, thin layer. I think the gel would be much easier than patches too, especially on the thighs.
The only other thing I'm aware of to reduce scarring is laser treatment. No idea of costs for that, and they may not do it until she has stopped growing, but something to consider.