i was never regular as a teen. never ever. it definitely can happen for reasons sometimes unknown. i agree about checking with her doctor, and about figuring out whether there's a chance she could be pregnant.
Sometimes when they are physically active and loose a lot of body fat, the menstrual cycle can space out or even stop completely until they slow down or gain weight. Also, if her eating pattern has changed, ex. dieting, this can affect her cycle, too. Of course, one would have to be sure that she isn't sexually active. So, hopefully you have good communication with her. But, I would say that it probably is the activity or diet. Check with a dr if you have concerns.