This is a difficult choice but there are some medical facts to help you. The surgery itself should have no effect at all on the dementia or Alzheimers.
The question about surgery is much more difficult to respond to because it is urological problem rather than a psychological problem. One thing you can do is ask for a second opinion from another urologist.
There several non-invasive alternatives, and one of them is hormone treatment, which may have( you will have to check out) some positive effect on the vascular system.