Is my mom suffering from depression or complicated grief?

Depression and grief do not have the same symptoms or course.  Depression is marked by negative thoughts and feelings about oneself - low self-esteem and self-worth. Grief, on the other hand, is marked by distress over the loss of another or, in severe cases, overwhelming separation anxiety. It sounds like your mom could be suffering from the more severe form of grief, which is also called complicated grief.   

In normal grieving sadness and bereavement is followed by resignation and finally, adjusting to the new situation.  In complicated grief there is searching, yearning, preoccupation with thoughts of the deceased, crying, disbelief regarding the death, feeling stunned by the death, and lack of acceptance of the death.  It is more common when a partner from a long-term close relationship is lost.

People suffering from complicated grief experience impairments in global functioning, mood, sleep, and self-esteem, and even though this sounds similar to depression symptoms it is distinct from it and requires specialized treatment.

Complicated grief responds well to a type of CBT which incorporates grief and loss counselling, where the grieving person can  stabilize, explore, and confront the most painful aspects of the loss, and then integrate and transform their grief.

I'm not sure what it is that makes you believe your mom is struggling with spiritual issues - if it is that she no longer attends church, this may be due to the social isolation that is part of complicated grief - but a Christian counsellor may be the one to help her if she wishes to integrate her faith into her process of recovery.