My grandpa (97) has recently been accusing my grandma of sneaking a man in their house. Is this a sign of dementia?

I am so blessed to still have both grandparents and both is relatively good health. My 92 grandmother and his bride for 60+ years of sneaking a man in their house. He breaks out in tears and gets angry that no one believes him. Is this a sign of dementia? My mom and her sisters are at a loss of what to do to help them. This is hurting my grandmother to constantly be accused of this. He talks about it for hours on end.