Until I started on the pill me period was extremely irregular. I dutifully counted out 28 days every month, but started using panty-liners and pads two weeks before and didn’t start worrying until two weeks past that point(I went through a lot of pads). Fortunately, it only lasts three-four days so I got some relief. Now I’m every three weeks and except for changing over to a European prescription when I moved there, I could almost predict it to the hour.
Not enough to panic over, but it should be weighed with any other risk factors she may have.
Early onset of menses, never having children, and not breastfeeding when you do have children can all increase your risk of breast cancer, but really, it’s not a death sentence. Many, many women starting menstruating early and never have children and never have breast cancer, either. There are women who menstruate late, have children, and breast feed who do get cancer.
Risk factors you can actually do something about include a high-fat diet (bad), obesity (bad), tobacco use (bad), radiation (avoid), alcohol (avoid or use minimally) and shiftwork (why I haven’t a clue).
According to some sources, exercising 3 times a week can lower a woman’s risk by 40%. So can getting adequate folic acid.
In other words, there are a lot of factors at work. Getting appropriate screening (self-exam, mammograms as appropriate) is really your best option for avoiding Bad Outcomes.