Clarify with the carrier: they do not store text messages on their server once the message is delivered to the recipient.  Make sure that you get that in writing because some carriers do indeed store them on their servers and archive them on heir websites if you are signed up for that service.  It is true that it is not a uniform thing across the board with each carrier.  There is another alternative.  You could subpoena the sim card from her phone and obtain a sim card reader that can read deleted text messages.  They are available to purchase.  Just go on line and look them up.  See if the court will allow this.  Good luck.