Causes of hair loss???? Help!?

I have a friend who's got a bald patch at the crown of her head, and it seems to be getting bigger. What could it be??? I'm worried about her because she never looks very well. She's 18.Causes of hair loss???? Help!?
stress %26amp; probably a poor diet :: tell her not to use shampoo with sls or als in it because it's been found to cause hair loss :: hope everything works out!!