If you want to change your friend's Snapchat display name back to their default, follow these steps:
Open Snapchat: Launch the Snapchat app on your device.
Access your friend's profile: Tap on the "Chat" icon at the bottom left corner to open your list of conversations. Find the chat with the friend whose display name you want to change.
Open their profile: In the chat conversation, tap on your friend's name or Bitmoji at the top of the screen. This will take you to their profile.
Access settings: On their profile, you will see a settings icon (a gear or three dots, depending on the app version). Tap on this icon to access their settings.
Edit their display name: In the settings menu, look for the "Edit Name" option. It might also be labeled as "Edit Display Name" or something similar. Tap on this option.
Remove the custom display name: If there is a custom display name set, it will be shown in the text field. Delete the custom name and leave the field empty.
Save the changes: After clearing the display name field, tap "Save" or "Done" to apply the changes.
By following these steps, you will have changed your friend's display name back to its default setting. Now, you will see their Snapchat username or the name they have set on their Snapchat account, as per their settings. Keep in mind that you can only change the display name back to default on your end, and your friend will still see their chosen display name on their account.