i want to stop iMessages from being deleted
There is no way to disable that function.
But you always can get another Apple device, such as mac or any of iOS family and login with her apple ID in iMessage section of settings.
Once you'll made it that will allow you get the copy of all the iMessage conversations.
Despite the reason of your idea, I just want mark it as not so fair activity. Check twice if you will be charged for that by the law of your country.