Help Mom feel needed.
If Mom was Dad’s primary caregiver, she likely spent most of her time with him. His death may mean she is left with a void in her life. As C.S. Lewis said in A Grief Observed about the passing of his wife, "Her absence is like the sky, spread over everything."
Mom may be at a loss on what to do with her time. She may feel like she is no longer needed. It is important to help her realize that she is valuable and needed.
Encourage Mom to get out of the house and spend time with friends and family. Help her get back into her hobbies. Go together to volunteer in your community. The more Mom interacts with others, the more she will feel appreciated and supported.
Mom’s house is full of memories, and it can be nice to get away from them for a while. When she is at home, ask if she would like company so that she doesn’t feel alone.