HOHO buses are not good transportation IMHO. I would Plan a trip with a focus on maybe one thing in the afternoon and one in the morning with great flexibility to just bag the plans if that seems good. There is a bus tour by Foxity that showcases the top sights in two hours; you can also do a HOHO with them for a bit more and that might be worthwhile. There are a couple of HOHO companies and you could do that but stay on the bus and use it as a tour.
Another way to see Paris and some out of the way spots she might not otherwise see is to do the canal boat tour that starts at Arsenal near the Bastille and wends it way through canals to Bassin de Villette. Part of it goes underground but mostly it is through neighborhoods you will not otherwise see and it is sort of fun to go through the locks along the way. You can take a nice picnic and bottle of wine and enjoy the day with little energy expended.
Museums like the Louvre have wheelchairs which she may not want to do, but it is something to know and in spite of the fact that the Louvre is an old chateau with tons of stairs everywhere, there is an elevator for every staircase -- so whether on foot or wheelchair she need not climb stairs. You get good at spotting the signs, but if you don't a guard can direct you to the spot around the corner where they are hidden.
The buses in Paris are designed for people with mobility issues; I would avoid the metro which is filled with stairs (a couple of lines have elevators -- 1 and 14 that I know -- but most involve lots of stairs) The buses kneel, they have wheel chair space and people 75 and over are entitled to have seats. You may need to be prepared to request that on her behalf so that the teenager flopped down in the reserved seats will yield them up. You can buy a booklet of bus maps at a news stand that makes planning easier. And the stops themselves are very well signed so you can see the route. Get Navigo Decouverte passes so that you can hop a bus without worrying about tickets even for short distances.
I traveled with my mother with mobility problems in Italy when she was 80 -- one issue that is true also in Paris is that every bathroom in a cafe or restaurant seems to be up or town a narrow flight of stairs, so using facilities in museums or the portable restrooms on the street when you see them is wise.
The other thing that was important is realizing that those sidewalk cafes are designed for your pleasure. Sitting for a couple of hours with a creme and a pastry or whatever and watching the world go by is restorative and an expected thing to do. One of the best 30 Euro I ever spent was on two giant gelato bowls at a sidewalk cafe at Piazza Navona in Rome -- overpriced (this was 15 years ago) sure -- but worth every cent. The waiter was gracious (Italians and perhaps the French too love to see an elderly mother being tended to by an adult child -- we had lovely treatment everywhere which thrilled my mother) and we watched the world go by in this lovely place for a couple of hours with our very tasty collection of sorbets and gelatos.
Have a great time. The memories are sweet; one of the best things I ever did for my mother and so glad I managed to do it while there was still time.
janettravels44
www.janettravels.wordpress.com