How to tell someone with dementia about spouse dying

Looking for guidance/advice. My grandfather has dementia and my grandmother is dying of cancer. How do we go about telling my grandfather. Do we avoid telling him? Do we tell him when she passes away. We aren't sure how to go about it. He won't remember or maybe he will off and on. But he will notice if she suddenly stops coming around. He is currently in a hospital bed and she is still at the nursing home. She comes visit everyday but eventually that will become too much for her due to her pain level she is experiencing. The cancer has spread and most likely wont be here by end of the year. We are at a lost for what to do in this situation.