I'm guessing the issue here is that you're both wanting to take advantage of the free 48-hour restart, and that is only possible with an iOS or Android smartphone / tablet with the app installed. So, that wouldn't be possible for your friend if she is using a physical 72-hour pass.
1) Does anyone happen to know if I can put both her and my cards on
the app on my phone?
Yes. When you click the "buy" button, you choose the number of cards you want and provide ID info for each pass holder. Now, if the passes are only on your phone, this means that you'll have to do all of your sightseeing together, at least for attractions you'll be using the card for.
https://www.firenzecard.it/en/shop/acquista_card
If she intends to use a physical pass for the first 72, hours, the website says this about "buying" the re-start:
Firenzecard Restart can be purchased only online at www.firenzecard.it.
To purchase it, you must enter:
1) the number of your Firenzecard that you wish to reactivate;
2) the email used to purchase the original Firenzecard online or purchase date in case of purchase at a sales point in the city.
So, it sounds to me like with just her card's # and purchase date in Florence, you could indeed activate an extension for her. Aside from all of this, you are aware that you need make a timed-entry reservations for the Uffizi, Accademia and Brancacci Chapel even if using the card?