Hair loss can be an indication of protein deficiency. I have had many students (primarily girls) who, in an attempt to lose weight, have suddenly decided to go vegetarian or strictly limit their in-take of meat. While they understand that meat tends to be high in calories, they don't usually understand enough about being a vegetarian to know that they still need to replace that missing protein, and that there are lots of other foods with protein that are not meat.
Additionally, teenagers are notorious for skipping breakfast and/or lunch or eating nutritionally-poor lunches. I can't tell you how many teenagers I've seen who eat a bag of chips and drink a Coke and call that a lunch.
Hair treatments--especially extreme ones--can cause damage to the hair sometimes causing breakage. If her child likes to experiment with his/her hair, it may be time to back off for a little while.
A hormone imbalance could be the cause. It seems rare in teenagers, but teenagers are hormonal creatures.
Finally, hypo- and hyperthyroidism both include hair loss as a symptom.
If it's none of those things, then she really should have her child checked out. It could be a sign of something more serious than any of the things listed above.