Sorry ummmm, I didn't comment on this earlier because I think in these situations there really no wrong or right, everyone is different. I hope your Nan will be ok and the news doesn't affect her badly.
I have just read this thread. When you say "they're telling her", I assume that you mean family members? Perhaps they feel that they have discharged their responsibility by telling her? Personally I would not tell her as it may have an immediate adverse effect and she may not remember tomorrow. I empathise with you.