There is no such thing as preventative chemo. She had a major surgery with many days recovery. That very likely was what is called debulking - generous cutting to remove a tumor and nearby tissue including lymph nodes. And after healing from the surgery, chemo is next. This is standard program and usually not the start of a survival story.
Let’s hope I am wrong, but what they are saying makes no sense. You don’t have major abdominal surgery with weeks of recovery for a cyst or a polyp and then find some minor cancer later that you electively treat with chemo. Something major was already underway.
Hoping the best for her and her family - not a royal fan but this is devastating news for any young family.