Admin Can't Delete Standard User - asking for standard user's Apple ID PW

Hello, someone already asked this question here but never got an answer that worked: cannot delete user, it keeps asking for p… - Apple Community

The situation:

I am the admin on my Macbook Air.

My ex is a standard user.

When I try to delete his standard user account, Apple asks me for his Apple ID password "to turn off Find My Mac". I don't have his password and I'm not going to ask him for it. I don't want to remind him I exist.

How do I remove him as a user?

Like the previous person whose question I linked to, I do not need standard instructions on how to remove a user. I am following these steps:

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.

But I'm getting stuck at step 3 when Apple asks me for his Apple ID password to turn off Find My Mac. I tried entering what I thought might be his password, but it didn't work so Apple canceled the process of removing him.

I've logged into the standard user/ex's account on this computer to make sure Find My Mac is turned off; it looks like it is, so I'm not sure why I'm getting this error.

Side note, even though it appears Find My Mac is turned off on his standard user account, does this mean it might actually be turned on so he can stalk me? He's that kind of guy, which is why this is really frustrating and frankly, a little alarming.