How can I get my mother to stop grieving over the loss of her husband?

My 96 year old mother lives with my husband and me, and our 2 dogs. She is in good health except for back pain and does take an anti-depression pills. My father died 24 years ago and she cries and says how much she misses and loved him.. almost every day. We live in the house I grew up in, but it is completely different than when they lived here 30 years ago. She can not see to do much but watch tv and read with magnifiers. We live int he country, and in the winter can not go out much. However, she is this way even in the summer. What can I do to help her move on?