Hello, I had major placenta previa. How far on is your friend? There is a good chance of the placenta moving if she is only 20 weeks, but if she is further on she might have to resign herself to the fact that she will end up with a c-section.
The risk of bleeding/hospital stay is about 1/4. Placentas can bleed unbelievably heavily so there is a real risk of serious blood loss, with the obvious health risk to the mother. The other serious consequence of heavy bleeding is early delivery of the baby, which is why if you bleed at all they tend to keep you in hospital to ensure that a) you are never far from a blood transfusion and b) the baby is not far from a neo-natal intensive care unit.
The risk of hysterectomy is associated with placenta accreta (which is very occasionally linked with placenta previa) which means that the placenta has grown into the lining of the womb, more likely if your friend has had a previous section. Overall though accreta is very rare so it is unlikely she will face this.
The most common outcome of major placenta previa is a short hospital stay (1 week/10 days) from about 36 weeks, no bleeding and a straightforward section, healthy baby, hunky dory!
Having said that...
I had my first bleed at 29 weeks and then was in hospital until dd2 was delivered by ceasarean at 36 weeks. It was monumentally boring and I missed my dd1 and dh dreadfully. I had regular bleeding, was prepped for sugery 2 or 3 times a week, but happily made it through to a decent gestation. It was discovered during the section that I had placenta accreta and I suffered serious blood loss (7 litres) had to have a massive transfusion and a second surgery but didn't have a hysterectomy although it was a close run thing. I woke up in intensive care and was seiously incapacitated for 3/4 days. DD2 had serious breathing dificulties (despite being 36 weeks) and ended up on nicu/scbu for a week, was tubefed for the first 2 weeks or so. However, we are both absolutely fine now!
I say this just because I never believed that I would have any of the serious complications because they are statistically very unlikely. Nevertheless it does happen to some people so whilst your friend shouldn't worry herself she should at least be aware of the possibilities.
BTW my consultants told me that they no longer think that there is a risk of the baby growing more slowly, the real risk to the baby is premature delivery.