Is she on the 28 or the 21-day kind of pill? Really they're the same except the 28-day packs have 7 dummy pills for the days when you have your period... some have lower doses of the hormones in the dummy pills, too. Maybe she should change pills -- there are several forumulations out there. Some people react better to some than others. If she's really taking them as directed there should be no irregularity of more than a day either way: after stopping taking the active pills, in a few days bleeding should start.

The way the pill works, it doesn't let you ovulate. So really, the bleeding that occurs on the dummy-pill days (or the days without pills for the 21-day packs) isn't a "real" period: there's no egg to flush out. Pharmaceutical companies really just have those off days to a) let you know you're not pregnant and b) make you feel like a normal woman. Technically one could take the active pills every day and not have a period: I've done it once (for my honeymoon) though, of course, don't do this without consulting your doctor.

Hope that helps? She should talk to her physician if you're worried, and see about changing to a new brand of pill.