They can, and I guarantee you the insurance companies will look at it that way. Hers is leaving out a huge chunk of the story: money.
It will probably be called an "elective procedure" and especially if there is anything cosmetic about it.
My mom had a melanoma removed from her eyelid and the costs were staggering. She had some cosmetic reconstructive surgery done to allow her face to continue to look normal. That's all: normal.
They had to fight for weeks to get a good percentage of it covered—or any percentage of the cosmetic portion.
I don't know entirely what the case is for a preventative mastectomy, maybe if your cancer risk is high enough your insurance will see it as a overall benefit to them, but it can't be easy or cheap to deal with at all.