Been there. Done both.
Assuming that her flight from Milan is non-stop back home to the US, she will need to be at the airport 3 hours before departure. Note that if she has a European connection, she might not need to be at the airport so early.
She could stay at a hotel within walking distance of Milano Centrale and take the express train to the airport, BUT with an 11:00 departure she would still need to get up at 0:dark:30, get ready, have breakfast if the hotel serves it early enough, check out, walk to the station, buy a ticket and get on the train. I've done this staying at the Hotel Berna, but my flight home left in the afternoon, so I wasn't so pressed for time.
If she stays at an airport hotel, or one nearby with a shuttle, it might be more expensive, but she wouldn't have to get up so early. I've also done this and stayed at the Moxy at the airport.
Something she might be able to do is to buy a round trip Frecciarossa (fast train) ticket when she arrives at MXP or Milano Centrale and make the return from Florence later in the day on Sunday, 11/12.
She can do the same thing for the Malpensa Express between MXP and Milano Centrale and save a few €, but it must be done online. See last paragraph below.
Note that on the Trenitalia search site, you can limit to Frecciarossa trains by clicking on Main Solutions and choosing FRECCE. That focuses on fast trains only.
Also please note that even some of them have one train change. There's minimal extra time involved, but the potential for confusion with any train change is great, especially in a jet-lagged state. She should choose an option without any train changes.
Here's a link to the Trenitalia search site in English:
https://www.trenitalia.com/en.html
Here's a link to info on buying tickets between MXP and Milano Centrale in person at MXP. Scroll down to see a round trip discount option. It's only possible online, but a ticket agent might be able to help her with that.
https://www.airportmalpensa.com/airport-train.php
Edited to add: On Saturday, 10/12 There are many fast train options from Firenze SMN to Bologna Centrale that take about 40 minutes with no train changes. Similarly there are also many fast train options from Bologna Centrale to Milano Centrale on 10/12 with no changes. Those take between 1 hour and 1 hour and 40 minutes, depending on how many stops there are along the way. Click on details to see where the stops are.