We were in cabin 6134 on deck 6. It was a great outside cabin, very quiet at all times even though on the deck plan it looked as though we were under the theatre. The only very brief noise was the anchor going down in Cannes, which was the only port where we were tendered to shore. All inall a great cabin in a great location.
You don't seem to be sure exactly which deck your Mothers cabin is on, but for her sake I hope it is deck 5 rather than 6. Deck 5, in addition to a large number of cabins, also has the main reception on it. This deck will also probably be where she will get on and off the ship in port, both of which can be quite convienient. There is also a couple of shops and a bar on this deck. All at the centre of the ship around the atrium. Deck 6, although not having anything other than cabins on it, is unfortunately situated below one of the main activities decks, with restaurants, bars and extertainment lounges, etc on it. This can sometimes mean that there is activities going on right above your head into the early hours of the morning, especially if you are unlucky enough to be under the theatre! It really does pay to sus out the cabin plans before you book and try to avoid any that are likely to be noisy. I hope that your Mum has a great cruise anyway.