It's unusual for a 15-year-old to be dealing with hair loss problems unless there is an underlying health problem. Of course things like what she's using on her hair right now might also be the problem. Things like hair dye, hair spray and even the wrong shampoo or conditioner can cause hair to get thin. Most shampoo you buy today is nothing more than detergent, it's like washing your hair with dish detergent. And most conditioners are nothings more than applying a plastic coating over your hair. You might want to check out shampoos and conditioners from your local health food stores.
If you don't think it's anything from what I mentioned above you might look into a vitamin or mineral defeciency. Teenagers tend to have a poor diet. I'd at least have a blood test to see if she is dealing with any of these issues.
