Rome in the middle of a business trip

Wow. Right on the Via Veneto. 3 blocks north to Villa Borghese Park and 5 blocks south to the Trevi Fountain.

What does she like to do? If she likes to walk and likes art but not crowds, she can walk from Via Veneto all the way to the Pantheon and Piazza Navona, passing the Trevi Fountain on the way. In P.za Navona she can see the fine museum in Palazzo Braschi (or the equally lovely Galleria Doria Pamphilj a few blocks away.) La Rinascente (department store) is nearby on via del Corso so she can have a look there on a Sunday. There's a wonderful restaurant at the south end of Piazza Navona (the other side of Pal. Braschi) called Cul de Sac where I am sure she would not be uncomfortable eating alone. If she is really a walker, she can continue on up via del Corso to Piazza del Popolo. Makes for a couple of hours of strolling and the opportunity to wander without schedule simply enjoying the city along the way. Sit in the cafe where Fellini sat, have an over-priced cup of great espresso, and enjoy the scene: priceless! The Borghese Gallery is open Sundays, and requires an advanced reservation about a week in advance. It is a 15 minute walk from her hotel. Of course the Colosseo, Foro Romano and Palatino are open, but that's an ambitious tour, IMO. I'd opt for a long stroll, a nice simple meal in a lovely cafe, and one of the smaller galleries where there aren;t hoards of people.

I'd suggest the Borghese Gallery. It's a remarkable collection in an interesting building, and it's a pleasant walk from her hotel. It should be less crowded/chaotic than other museums might be on Sunday, since admission is strictly timed/controlled. However, she will need to make advance reservations, and now would be a good time to do that. The hotel should be able to do that for her. She might also want to consider a walking tour, maybe even an evening walking tour for Saturday night. Context Rome has good ones, with only 6 people per group, so there's lots of flexibililty. You could check their website for offerings.

Love Cul du Sac on Via del Governo Vecchio. A nice walk would be to visit the Galleria Borghese, then walk through the park to the Pincio for a view of Piazza del Popolo below and most of Rome surrounding it, then take the footpath down to Piazza del Popolo and continue with other posters' suggestions. If she gets as far as Piazza Navona or Campo dei Fiori, bus #116 (an electric mini-bus) will take her back to Via Veneto. It also stops a short distance away from the Galleria Borghese.