Her statistical chance of accreta with previa present at delivery depends a lot upon her history. Has she ever had a c-section? D&C? Abortion? Any other uterine surgery? For every 'yes' to those questions, her odds go up.
I've had two sections, nothing else on the risk factor list like D&C or abortion, and if my PP had persisted til delivery (thankfully it did not), the chances of accreta were between 40-50% depending on which study you read.
There are also u/s findings that can indicate possible accreta, so I'd ask the doc if any of these signs are present. Depending on the answer, I'd ask to be referred to someone capable of doing a color doppler u/s, as this seems to be the best according to research in detecting accreta prior to delivery.
IF she DOES have this condition, it is VERY VERY SERIOUS. Truly, it is. There is a 7% overall maternal death rate. Hysterectomy is often the ONLY way to save mom's life...literally. And if u/s findings indicate accreta prior to delivery, most research I read indicated leaving the placenta intact within the uterus and going straight to hysterectomy was the safest option.
Now, going straight to hysterectomy is easy for me to suggest cause we are SO done with my uterus. I don't give a hoot one way or the other if I come out of this pg with a uterus or not. Obviously I'd prefer to have nothing go wrong at all, textbook surgery and recovery, blah blah blah. But there are to be NO heroic measures to save my uterus if I am bleeding out. None whatsoever.
If she is not finished having kids, she will likely want them to at least TRY to save her uterus. But the bottom line is, blood loss with accreta is MASSIVE, as in your entire blood volume plus some in the average case, meaning you are getting your entire blood volume plus replaced. DIC is very common with accreta, another life threatening complication.
Actually, from what I read, accreta now leads the pack for cause of maternal death during c-section. So if her doc truly suspects she has this condition, I'd be gathering as much information as possible, asking for a referral for better u/s, etc. It's not something to play around with at all.