What Causes Intermittent Menstruation?

My friend is on the pill and has been for quite some time now, but just recently when she is taking her period, she is bleeding for around 5 days, stopping for about a week and then bleeding for over 5 days again. This has been happening for the past few months now. Should she consult her doctor or is there nothing to worry about? Please help as I am quite worried about her.