Hi Herf,
I'd like to offer some suggestions based on my experiences.
First, congratulations to your wife for keeping loyal to her weight loss since last year (and wanting to stay healthy in this new year.)
No worries for working off any calories consumed. Unless mobility is an issue, you can walk "a lot" in Rome. Since she is on WW, she is aware of her options.
Rome is not all about pasta. She can order a meat dish (chicken, pork or fish) with a vegetable (low in carbs) - no potatoes or get salad. But, the amount of side potatoes they serve in Rome is nothing like what we experience in America - "double sized everything!" Maybe she can skip a fancy dessert and have an espresso or sip Limoncello.
If interested, I can give you the name of the restaurant in the Termini area. I realize it's not close to the Colosseo area. I have no suggestions for the area you are staying in. Or, from my suggestions below, try to find a restaurant similar to the areas you will be in. There is no obligation to stay at a restaurant if the prices or choices are not compatible with you and your wife.
For me, the restaurant I frequented (close in prox. to Termini, where I stay), has two locations about two blocks from each other. They are very, very busy - with locals, business people and travelers. They operate from 12:15 PM to 11:00 PM.
They are a family-style, sit-down, tablecloth, inexpensive restaurant with a buffet bar of assorted veggies, spinach, mushrooms, chicken wings, etc. - didn't try it, but many people did. And, it's not an "all you can eat" buffet - you purchase a small or large plate - and "fill it."
Off the menu, their roasted chicken w/veggies was a popular dish; as I saw it come out of the kitchen often. A "regular" who ate there every night ordered his pasta dish, chicken, etc. A different pasta dish most nights but the chicken always. lol.
Most, if not all of their pastas are homemade. Their menu reflects what choice may be "frozen" ahead - a very small selection of dishes. Their pasta dishes are delicious but not overwhelming. I usually had a pasta dish w/wine or a pizza & "stop." A couple of times, I was extra hungry & ordered melone con prosciutto or carciofi (as a starter) - melon was firm & prosciutto not too salty - Artichokes were delicious. Then I would have dessert, usually tiramisu, which melted in your mouth, & espresso - "on the house." The waiters sometimes bring out a complimentary glass of limoncello.
Back to the business - It started to get busy with patrons 'around' 7:30 PM - travelers & locals alike. By 8:30-9:00 PM, there were no seats, waiting only. The staff do not rush you out the door.
One day, I walked by at "lunchtime" - about 1:00 PM and it was standing room only. One of the waiters saw me and waved me in to say hello. I was not there for lunch but observed many business people dressed in suits, etc. (and some travelers or casual locals) having lunch.
So, it's very possible to find a little restaurant that is not touristy, reasonable prices and a home-town feel, with varied choices for many people.
Anyway, with the complimentary buffet breakfast at my hotel, I had eggs, fruit, sauteed zucchine, (maybe a cornetto), espresso x 2, a bit of cold cuts, cheese, and cherry tomatoes. So, there are choices of healthier or lower carb eating.
I would have a snack bar for "lunch." I was not hungry until about 7:30 or 8:00 PM.
If your wife wants to treat herself to gelato, maybe she can skip the cono and order a cup “coppa” (KOH-pah). Many a gelateria offer different sizes.
Ex. La Romana https://www.tripadvisor.com/Restaurant_Review-g187791-d4492728-Reviews-Gelateria_La_Romana_Roma-Rome_Lazio.html is very, very good. I'm sure you can find others who are just as good.
Have fun in Rome! I love Rome and had many wonderful experiences and memories. I met so many nice people and made a few good friends :)