If it's an iPhone, you can get a screen capture by pressing the home button and the power button at the same time. Sometimes it takes a minute to get the hang of it, but keep trying. That will save an image of the page of texts to your photos, and then you can hook your phone up to a computer and print the images out. Like the previous answerer said, this means you would have to keep the messages on your phone for proof. Though I can imagine this is uncomfortable, better safe than sorry, right.