Poor thing - reassurance and bio-oil. Most men don't notice and they fade to practically nothing - look at Kate Winslets breasts in the nude scene in Titanic - the silvery lines are there and she is beautiful.
Exactly the same thing happened to me at 12. My Mum took me to the doctor and I was mortified. Nothing to do, but mine have now nearly disappeared to fine silvery lines. Elemis oil is very good, or good old bio-oil, but lots of reassurance.
They will go white/silver and be less noticeable but I'm afraid nothing will get rid of them. Bio Oil is great but it's for prevention, it doesn't reduce the appearance of them once they are there - nothing will. Self tan or tan make up works well for blending in white/silver marks (which don't tan, so will show more when she has a sun tan) and concealer make up will cover red or purple marks. Tell her that nearly everyone has stretchmarks by the time they are an adult, so they are totally normal, and so are scars, age spots and wrinkles! I had stretch marks on my stomach and hips by the time I was 14, but the positive side is that the same ones re-stretched when I was pregnant, so I completely avoided getting any new ones!