My twins were MOMO - mono-chorionic, mono-amniotic. As Leo says it is very risky and very rare. Apparently it's all due to when the egg splits...early on and you get id twins that share a placenta but not a sac, later on and they share a sac as well...any later and you have conjoined twins. (I was told that by the sonographer at my 20 week scan.)
Mine were also born by CS, but at 37+4 weeks; they were both head-down from early on and I had been told I could try for a natural birth if I went into labour spontaneously so long as there were lots of medics around to help! But I was quite happy with the CS.
My DTs were 5lb7 and 5lb12 and are now happy, healthy 20 month olds. As throckenholt says, any danger from them being mono-mono twins is now past.
if your friend wants to chat, would be happy to. Best wishes to her, hope her DTs are healthy and home soon.