My MIL seemed so much worse after FIL died. But part of that is that they covered for each other -- both of their memories were failing, but between the 2 of them, they could keep track of things better.

My grandmother did well for quite a long time after my grandfather died, but she was living with her son (my uncle). She taught me card games and trounced me regularly. When her health declined to the point where she needed to go into assisted living/nursing home, her mental state deteriorated quickly.