Do no harm. Whether your grandma's faith is based in something real or not, her belief that it is provides her comfort, just as her belief in the sincerity of your prayer will comfort her in her final hours. In the grand scheme of things, a small lie made out of love is not such a terrible thing. When the time comes, it will be about her, not you or anyone else.
I'm sorry about your grandma and hope you are ok.
I'm sorry about your grandma and hope you are ok.