Taxi from Porta Garibaldi in Milan to MXP

She should probably get to Milan the night before her departure
Too risky otherwise unless she is OK missing her flight
What time is her flight and where is it headed ? Direct to US or thru another European city ?

Taxi is fixed rate something like 130 euros but the amount of time it will take depends on time of day and traffic

She can take a fast train Florence to Milano Centrale (doesn’t need to go to Porta Garibaldi but then still an hour on Malpensa Express to MXP-runs every 30 min)

If she takes the train on the same day of the flight, it is impossible to arrive to MXP on time if the flight is before 2:30pm.
It is risky even if the flight departs later. Delays happen. Yesterday for example high speed trains recorded delays of nearly 3 hours due to technical issues between Bologna and Milan.
Tell her to go to Milan the night before and spend the night in Milan or near Malpensa.

As ChristineH asks, the flight time and destination are important pieces of info.

If you do the math, 2 hour train from Firenze to Milano Centrale, guess of 15 minutes
to wait for the second train, 1 hour for the second train, need to be at the airport 2-3
hours before the flight. If this is a transatlantic flight back to the US that leaves in the
morning, there is no way she can get there leaving that morning and make the flight.

Even if it's an afternoon flight, she needs to be on the train out of Florence 6-7 hours
before the flight departure time.

Going the night before and staying in Milan or near the airport seems like the only
(not even the best) answer.

Doing that will also split the travel into two manageable parts instead of one really
long day.

I think you will find that there are more trains to Milano Centrale from Florence, and you can get the train to MXP from there. There is a train every 30 minutes, at 25 and 55 past the hour.