My sympathies.
I presume the reason she is not being given HRT is because of the ovarian cancer, and the treatment she is probably still having for it.
She should ask for it to be explained carefully to her.
She should, as said, not take any herbal / natural / whatever remedies - complex reasons, but in short, the substances may either disrupt any cancer treatment, or precipitate specific problems. Please be very careful with this.
She will, sadly, have quite a bad time, because it is a sudden disruption of hormones, as opposed to the gradual reduction of the 'normal' menopause. It may be helpful to see this as a side-effect of the treatment that has saved her life.
Her medical team, and especially the nurse, should have some helpful advice to manage the symptoms - as described by Eglantine.
My best wishes to you and your family