They could be sporadic her whole life, or she could just suddenly snap into a regular cycle; it's really different for each girl. Generally irregular periods are not a big issue unless you're older and trying to get pregnant yourself.
The simple answer is that she should talk to her doctor. If she's old enough to be menstruating, she should have her first pap smear.
It can be a sign that something else is off such as,
- Hormones (there are a lot of chemicals in our environment that can throw off hormones. Just a random example, antibiotics and or growth hormones in dairy and meat can throw them out of whack, as well as flame retardants such as those on bed sheets.)
- Low body weight (the body needs a certain fat content to menstruate)
- Very high body weight (fat can change hormones and throws them off)
- Poor diet and not enough vitamins/minerals
- Lots of exercise (I knew athletes in high school that didn't menstruate while they were actively practicing for sports)
A really easy fix to this (again have her talk to her doctor) is the birth control pill because it will regulate all of her hormones and guarantee that it will come every 28 days.