Unless you BLOCKED texting, the phone would still be RECEIVING texts (from her friends, alerts she may have had set, etc).even if she isn't answering. Whether they are opened or not doesn't matter - if they were sent to her they are counted as received messages.
Are you sure they are being SENT from her phone? Or are they received messages? If they are sent, possibly she got hold of a friends' old Verizon phone and activated it with her number....
And to answer your question about billing based on previous usage, NO. The billing is based on actual usage. Check the details online for numbers texted to and from, and find out whether they are all received messages, or sent and received. If you have the phpne in your possession and she is still sending texts....find out form her HOW.