Actually, she can, breast cells are not limited to the breasts, neither are ovarian cells limited to the ovaries, hence the 5 - 10% remaining risk.
Woman who have this procedure cannot expect to never have to check for ovarian or breast cancer again, it is not a 100% preventative.
She can't have breast cancer if she doesn't have any breasts anymore!
What she is doing is prevention. Like fixing the engine before the warning light comes up.