Dear Francesca,
Just so you know there's another of us out there with the same problem. My Mum has just not accepted that my Dad died a few months ago. Every time I told her, she got anxious and said "why didn't anyone tell me?". Now I've simply stopped saying that because I don't want to cause her more distress. I just say "he's not here, I don't know where he is at the moment, but don't worry".
Sadly her home is just down the road from the churchyard where Dad's ashes are buried, but I don't suppose I'll ever take her there.
Good luck.