I have uterine prolapse, and have worn a pessary as well. Once I got to around 13-ish weeks, I didn't need the pessary, since the uterus lifted up out of the pelvis. It feels SO much better now. I'm planning a vaginal, unmedicated birth at home, and I don't expect any problems with it. I had prolapse before my last baby, and he was born easily.
The only thing that would make a vaginal delivery unwise is if she has had repair surgery, and they used mesh to hold it in place. If not, she is a candidate for vaginal birth.