From Monterosso or La Spezia it is a 4 hour train trip to Rome....Day
trips from Florence to CT are not practical as travel will eat up
hours.
While it does take some time to get to the CT from Florence, with a longer stay there they'd have the flexibility to at least choose a dry, sunny day for a day trip. The CT is not a lot of fun in the rain, and especially not for hiking...which is what she'd previously expressed an interest in for a single full day/2 night stay there.
We've a friend who only had one day there and it poured buckets. Not a big deal, maybe, if hiking isn't in the plan but can otherwise throw a wrench in an outdoor plan. We planned 3 nights in the CT to give us at least a 50-50 chance of one dry hiking day, and it's good that we did as a train strike ate up most of arrival day!
There is a daily 7:36 AM train from Firenze Campo di Marte station direct to La Spezia that takes only 1 hour, 35 minutes: no longer than a fast train from Florence to Rome, or the time it takes from Rome to Pompeii. And it's inexpensive. From La Spezia it's mere minutes by train to the nearest of the CT villages (Riomaggiore), and very few minutes by train to cover the 7 miles between Riomaggiore and Monterosso (the most northernly village) and the 3 villages in between.
Going back to Florence, there a daily train that leaves La Spezia at 20:03 and arrives at Firenze Campo Di Marte 1 hour and 37 minutes later.
While I'd absolutely agree that it's a long day to do either the CT from Florence or Pompeii from Rome, choosing the day based on favorable weather may be preferable to relocating 4 people for one day and having Mother Nature decide to be uncooperative. :O)