We were there in February a few years back. A friend who is married to an Italian and thus travels there a lot warned me not to be fooled by the temperatures that were posted online. She said "it always feels cooler and damper than you would think"! She was right. 40 and 50 there felt colder than 40 or 50 here in the midwest!
One thing we noticed in Italy is that the shops, coffee shops, etc are NOT universally heated. So when you are in the midwest (we're in Michigan) you can hop into a Starbucks or pop into a store to warm up as you're walking. There you'll find the shopkeeper usually huddled in an unheated store in a heavy wool overcoat. Restaurants in Rome were usually warm, I think just because of the kitchens, but when you got outside of ROme to less heavily trafficked areas, we noticed that the warmth of a restaurant was directly proportional to how busy it was. Body heat of patrons + heat from a busy kitchen = a warm restaurant. We ate many dinners in restaurants in Sicily with our winter coats on!
Also, heating isn't always available in flats all day. We rented a flat in Rome and the heat turned on in the building once in the morning (starting at around 7 a.m?) for about 2-3 hours and then again in the evening (5 p.m. or so?) for another 2-3 hours. Then you had to count on the apartment holding the heat for the time inbetween. Our first day we didn't understand that and turned the heat down in the morning when we left (as we would do at home) and didn't return in the evening until after the heat had turned off for the day......so we missed the evening heat-turn-on and nearly froze our keisters off overnight!
Absolutely have her take layers of clothes. But she will want to have some fashionable things,too, because those Italian gals really dress up and put on the make-up. If she has a black leather coat, take that. Or a black wool peacoat or overcoat. But definitely take along her warm Minnesota coat, too. Polar fleece type gloves. Good walking shoes with thick soles (something more along the lines of the soles on Sketchers rather than tennis shoes, she'll understand that). The cobblestones are COLD in Rome and the cold seeps right through the soles of your feet. She will be doing a LOT of walking. Wrapping a scarf a couple times around your neck is as warm as putting on an extra sweater. She will be able to get some nice souvenir scarves, hats, etc in jaunty Roman fashion when she's there, but have her pack one of each also.
But tell your daughter she will also THOROUGHLY enjoy it! We had such a great time there in February without a lot of tourists around. It was wonderful and we really felt like it was "our" town.
I'm attaching a photo (this is my first attempt at attaching photos). These are my kids (goodness they were little) and really they should have been dressed warmer. They could have used hats, for sure!!! Notice their thick soled boots.