Bob is right. The cities are built for aesthetics and not accessibility. I use a cane in many places and with uneven walkways, cobblestone streets and many cities have very uneven and hilly streets and sidewalks, it can be a of a challenge to get around in these places.
The places I had the most problems were; Athens (Acropolis), some parts of Rome and Ephesus. I actually had the least problems in Egypt.
I wish I had paid more attention to see if and where there were handicapped access areas to major attractions.
I do think that a walker or wheelchair will limit your options but not eliminate them. If she does have a walker make sure it is one with wheels and brakes.