previously deleted texts reappear on text thread

I have a friend, we both have iphones. When she gets a text for an appt reminder or when you get a text verification, my friend will delete the text when finished viewing, but then next time she gets a reminder text or text verification her previous texts are there too even though she deleted them prior. My phone doesn't do that. Is there a setting or something to explain why all her prior text history comes back with a new text generated and mine doesn't?