Roberto's right - Florence has a lot to see. If she can't spend more than one night, she should get the earliest train she can from Venice, and the latest train back from Florence the next day. However, depending on where she's staying in Venice, it can take time to get to the station - allow for this. From central Florence, it's a short walk or an even shorter taxi ride back to the station, so getting a late train is not a problem.
Here's Rick's ideas for how to plan your time for a short visit: http://www.ricksteves.com/europe/italy/florence/planning-your-time
If you click the "At A Glance" tab on this page, you'll see his list of highlights (again, just his opinion, although he certainly covers the biggies): http://www.ricksteves.com/europe/italy/florence
If your wife wants to see the Uffizi or the Accademia (and I assume she does), she should get advance tickets (you'll see Rick emphasizes this on the pages I linked). On a short visit, she won't have time to waste in the (possibly hours-long) lines.