What is her diet like? I dont understand the pill comment though, because the only pills I know of "remove" your period for awhile, although many women after 6 months or so start bleeding till no end when the body adjusts and seems to build up a backlog. Alot of doctors dont seem to get the dosing quite right though so maybe her pill is dosed a little off.
If she is suddenly however playing with her diet there is a reason right there, removing fats etc from a womans body can throw the period cycle out of whack, although its usually starvation dieters that have this problem. If she is very regular though most of the time, and is suddenly off and has not been playing with the diet, then she should see a doctor if she thinks she has a reason to worry about it.