it depends on how low is the placenta. If it lies completely over the cervix, then must do c-section. If not, maybe no need. Only your doctor can tell you that. I had a low placenta on my second pregnancy but it moved up as the baby grows. I dont't remember when exactly it moved up. I read only 0.5 to 1% of placentas remain low-lying at the end of the pregnancy, which is not a lot. So fingers crossed for your wife!
The risk is that your wife may have bleedings (light or heavy). Go and see the doctor when it happens. To prevent that, tell her not to carry anything heavy. I dont't know if you have an other child. I stopped carrying my #1 when he was 13 mo. Whenever he wanted a cuddle, I let him sit on me.