Having an ovarian cyst is very common. My doctor told me that most ovarian cysts resolve on their own but to let nature take its course. They can be very painful as I am going through having one right now just under 2cm. I cannot even begin to imagine passing a 5cm one.
Ovarian cysts is just the ovaries' way of ovulating. Sometimes it is from the egg not pushing through the follicle and filling with fluid and sometimes it is from the egg pushing through and the sac doesnt dissolve as its supposed to and fills with fluid/blood which is what happened in my case.
Perhaps having your daughter put on birth control will even things out for her and she won't experience cysts as you are not ovulating when on birth control.
I hope you get all the answers you're looking for and update us if you feel the need to do so.
I suppose she's been to the doctor since she's been diagnosed as having a cyst. What did her doctor say? Is s/he going to wait and see if the cyst resolves on its own? At 5 cm it's just borderline, whether surgery is needed or not. Most likely it will resolve. Will she have to take BC pills to help?
I had many cysts when I was in my teens. Very erratic periods, pain, etc. My doctor put me on BC pills for a few months to even things out and that helped for quite a few years.