I was just informed I have a cyst on an ovary. I was alarmed because it wasn't there at all 3 months ago and it's nearly 3cm big. But the technician said it's not anything to worry about. She said it's common and most likely caused from the unfertilized egg passing through. Sometimes the egg sac (or whatever the technical term for it is) doesn't rupture and the cyst is just the remains of it. She said it'll likely be gone by next month.
Apparently it has to do with where you are in your cycle when they do the ultrasound.
Your friend should call her doctor and ask about it if she's worried.