Has she been on the pill for more than 3 months or so? I know when I started on my second month of using them, I had a period that lasted a full month.. It was however very light during the time I was not suppose to have one.
I've completely missed a period before once for taking 2 pills in a day when I missed in the last two weeks of the pack.. but that wouldn't make a period last longer.. I agree with the one above me.. have her see her dr. to make sure it's not from something else if she's been on it for awhile!
taking 2 birth control pills in a day wouldnt prolong a period trust me i was on the pill for 7 years i can't count on 2 hands how many times i did the 2 pills in one day thing.. she should get in to a doctor could be a number of things.. cyst.. who knows..