I would get a second opinion, N.. Look, I've had both kinds of ovarian cysts - those that burst and those that don't. The one that didn't grew to be 7 cms. I had to have laproscopic surgery to remove it. My surgeon told me that normally he can't save an ovary once the cyst gets that big because of the amount of bleeding involved. I got lucky that he could save my ovary. You don't want this to go on too long because the bigger it gets, the more possibility that when they finally go in and get it, they end up having to take the ovary too.
I just noticed that you said you were thinking of taking her to a gyn - why haven't you? That's the kind of doctor she should be seeing. This isn't a regular doctor's specialty.
Just to let you know, a ruptured cyst is up there on the pain scale of passing a kidney stone and giving birth. I pulled a muscle in my shoulder during one of my 2 bursting episodes. You will think she's close to having a convulsion, it will hurt her so much. This is another reason you need her to be followed by a gyn (and yes, use a woman, because she should not have to have her privates looked at by a male doctor at her age). If her cyst bursts, you should get her down there for an ultrasound right away so that they can see that it has burst. She will hurt for a full day or more because all that fluid in her abdomen causes lots of pain.
The truth is, as painful as a bursting cyst is, it's better for it to burst than for it not to burst. If it doesn't burst, it needs to be removed.
Lastly, this is hard to think of for a 13 year old, but good to talk to the gyn about. Cysts tend to keep happening. After going through the ordeals of them (3 with me), the thought comes of how to prevent them. There are birth control pills like Mircette that help prevent cysts. I stayed on Mircette for 11 years just for that purpose. A year and a half ago I stopped taking it because I am in peri-menopause now, and low and behold, I have another cyst on the other ovary (just 1 cm so far). Not too much pain, but I am having to have this one watched. I'm kind of tired of having ovaries now at this point in my life since they are a pain, but oh well!
Please get her to a gyn - it's really important.