cannot delete user, it keeps asking for password

Source of below >> https://support.apple.com/en-ca/guide/mac-help/mchlp1557/11.0/mac/11.0

Delete a user or group on Mac

If you’re an administrator, you can delete users that you no longer want to have access to your Mac. You can also delete groups you don’t want.

1. On your Mac, choose Apple menu  > System Preferences, then click Users & Groups.
2. If the lock at the bottom left is locked , click it to unlock the preference pane.
3. Select the user or group you want to delete, then click the Remove button  (looks like a minus sign) below the list of users.
4. If other users are logged in to this Mac now, you can’t select them.
5. Do one of the following:

• • Save the home folder as a disk image: Select “Save the home folder in a disk image.” This archives all the user’s documents and information so the user can be restored later if needed. The disk image is saved in /Users/Deleted Users/.
  • Leave the user’s home folder as is: Select “Don’t change the home folder.” The user’s documents and information remain and the user can be restored later if needed. The home folder remains in /Users/.
  • Remove the user’s home folder from the computer: Select “Delete the home folder.” The user information is deleted and the storage space is freed.
• Delete a sharing-only user or a group: Click OK.
  • A sharing-only user doesn’t have a home folder.

1. Click Delete User.

You've probably tried this procedure but if not have a look.

https://www.businessinsider.com/how-to-delete-admin-account-on-mac?r=US&IR=T#:~:text=How%20to%20Delete%20an%20Administrator%20Account%20on%20Mac,from%20the%20list%20on%20the%20left.%20See%20More.

Have you tried this: (it may not work but is worth a try)

System Preferences

Family Sharing

Family Sharing (under your name)

If your friend appears as part of your family click on Details and remove