Namenda is used for dementia and Altzheimers.
Although the tests came back showing no signs of Altzheimers it does not mean that the 80 year old lady does not have dementia. I would discuss this with the doctor.
Fluoxetine is another name for Prosac and this is an antidepressant. The doctor must have come to the conclusion that the lady does have depression for this to have been prescribed.
I would keep a watch on your neighbour and report any side effects to the doctor.
I would consult her doctor before stopping the medications. Some require a weaning off period, some do not. Her pharmacist may also be helpful.
Alzheimers is progressive, so she may start as simply forgetful but in the later stages one becomes completely debilitated until the disease makes someone cease to function.