Hi, Keith,
We just returned from Rome, and here is what I know as of July 1:
The Colosseum's exterior is scaffolded on one section (maybe a fifth). In no way does it detract from the overall majesty of the Colosseum. I wasn't disappointed at all.
The Arch of Constantine is beautiful, not scaffolded at all; in fact, a bride was having her photo taken there with the Colosseum in the background. So I think that's a big vote of confidence. :)
Fontana della Barcaccia--totally scaffolded, not really viewable at all. The Spanish Steps are fine, but they do have the scaffolded Trinita dei Monti at the top. So the view will not be the same.
I'm going to confess my shallow, mere tourist disappointment when we arrived at the Trevi Fountain and found it drained, scaffolded, and fenced off--and I have been to Rome before and seen it in its glory. If your mom is like my mom, then she's seen Three Coins in the Fountain and Roman Holiday, and the Trevi is a must-see. (Same with the Spanish Steps above.) I'm not going to apologize for it, and I don't even get the "I'm 88 years old" excuse. I do think if someone had told me in advance, I might not have been so sad about it.
The Four Rivers fountain in Piazza Navona cheered me up immensely. The fountain is beautiful, it's very accessible, it's on an incredibly beautiful square surrounded by charming (yes, touristy and expensive and probably not fine cuisine) restaurants. We had a lovely lunch overlooking the piazza and watching people.
The Pantheon was pretty fabulous, too. Talk about history....and don't miss Raphael's tomb. It's touching.
The Piazza del Campidoglio on Capitoline Hill is still mostly unscaffolded, and the view is great.
I never cared about the Altare della Patria (the Vittorio Emanuele II monument) before, but we took the elevator up (senior citizens get a discount) and the view of Rome from the top CANNOT be beat. And it's by far the easiest way for an 88-year old person to get a view like that in Rome. There is also an outdoor patio cafeteria-style restaurant on the back side of the monument with a great view of the Forum and the Via dei Fori Imperiali.
St. Peter's is amazing.
I know you have gotten some great advice, but I think the point is--she will treasure her trip with you, you will treasure the trip with her and the memories you make, and it will be wonderful. Best wishes.