Grieving can be difficult for anyone who has suffered a significant loss. It can elicit emotions and feelings such as sadness, anger or hopelessness.  In addition to this normal grief process, you must then factor in that your mother has Alzheimer's disease. The loss of your father--your mother's "anchor"-- is undoubtedly traumatic, making it difficult for your mother to cope with regardless of her diagnosis.  It is possible she is feeling insecure without him present, so it is essential she know there are people who will care for her in his absence.  The next time your mother is asking for your father, you can redirect her to a better state of mind by distracting her with something positive.  Some examples of this process would be to state, "I know you miss Dad. Tell me about your favorite time with him" or "I miss Dad too, and I know he knows you love him."  Another technique is to use photos such as from their wedding or other family event and reflect on past happy memories together.  Both of these ways will allow her to reminisce, while also providing her with the comfort and reassurance needed, related to his loss.