Unfortunately, WhatsApp does not provide any tools to prevent someone from deleting messages. However, you may be able to get around this issue by using third-party applications that are designed specifically for backing up chat history. These apps can save all your conversations and allow you to view them even if the other person deletes their messages. Here are some steps to help:
1. Search online for a reliable third-party backup app that is compatible with your device's operating system (iOS or Android).
2. Download and install the app on your device.
3. Follow the instructions in order to sign into WhatsApp using the new backup application and configure it according to its settings page - ensure that automatic backups are enabled so as not to miss out on any future messages!
4. Once done, all of your chats will now be securely backed up onto the server of the third-party application, allowing you access even after they have been deleted from within WhatsApp itself by another user(s).