From what I've heard, cloning a cell phone will let make both phones ring simultaneously and they'll both get the same text messages as they come in, but both phones can't pick up and talk at the same time. It wouldn't work like picking up an extension phone in your house where you can listen in on the other person talking.
There is software you can install on smartphones that will allow you to listen in on their conversations and texts. I don't know if it works in real time. I know that you need physical access to the phone to install it though. She's probably just being paranoid for no reason though.
Also, here's a cell phone tap that I did years ago:
