West Nile when it affects the brain does not usually cause dementia. It is most likely that your mother has encephalitis related to the West Nile. This should not be a permanent deficit. With time her mental function should progressively improve. When West Nile affects the brain or other parts if the neurological system it can affect the brain tissue itself or the layers that cover it viz the meninges. It clinically is difficult to distinguish these and therefore the term meningoencephalitis is used. In this situation the patient is confused and may appear to be demented. The symptoms are due to brain swelling and inflammation. As these resolve the patient returns to normal. Other neurological manifestations include cranial nerve abnormalities causing the patient to have impaired vision, facial palsy etc. Paralysis is another presentation as is a presentation similar to Parkinson's disease where the patient may have a tremor, the limbs get rigid, the gait slow with small steps and an expressionless appearance.
Some medications have been used with variable success in the treatment of West Nile fever.