Though you are not the person who should be giving the information to the patient, I feel everyone has a right to know that. Of course, as others have mentioned, there are a lot of factors playing in this...culture, family, POA-gaurdianship, etc.
In circumstances like this I always think, "Would I want to have this type of information withheld from me?" My answer would be "no". A person should have the right to make his or her end of life decisions and take care of their last wishes, will and property/finances, insurance, burial preparation, and final good-byes. This is beneficial not only to the patient, but to the grieving family and friends as well.
Once she is notified, then the right steps should be taken to support her through this time....chaplain, hospice, palliative care, counseling, etc. I would consult with your healthcare team.....other nurses, physicians, and designated family and friends involved in this patients care.
I had a similiar case where an elderly woman had terminal cancer. She was like an angel, so delicate, sweet, and kind. She was her own POA and yet everyone knew but her. The family did not want to tell her out of fear and not really knowing how to tell her. I thought it was highly unethical of the miscommunication of our healthcare team and her multiple physicians to let this go on. Good Luck!!