If your sister is still on narcotic pain meds, that is creating the constipation. It is vital to continue stool softeners as well as eat as much fruit as can be tolerated (tell your sister to add apples and pineapple to the mix, that may help-she can also try Activia yogurt). It does take a bit to get balanced after POP surgery, there will be much internal swelling but as she comes off of pain meds and continues the fruit and drinks lots of water, it should get balanced. She should mention it to her physician when she goes in for her checkups to see what the dr suggests.
And thank you for taking the steps to find some answers for your sister!