It very well may just be growing pains (sometimes it seems those things just refuse to quit growing!). Some people get them worse than others. I used to get them in my legs but other friends got the pains in their breasts. It could also be hormones, as some women's breasts get sore just before a period or during pregnancy. If you can rule out all possibilitiy of pregnancy, and she says the pain is more constant than her period, then it's just growing pains. If you are really worried that it's cancer (which isn't completely impossible) then her doctor would be able to evaluate any other accompanying symptoms and determine if more action needs to be taken. But really, I think she's just fine.