Can't reset system password for second user

@BDAqua, I just came back from working with the iMac and password issue. I was able to open the lock on my brother's account using my Admin password but it also required that I input my user name. Once my brother's account was unlocked, I was NOT able to change his password. It still prompts for the old password which is what he can't recall. I then logged into my account to try and edit his account and it did not allow access even when my account was unlocked. Once I logged out of my account, it brought up the screen showing both accounts for log in purposes and provided the button to change the password on his account using his Apple ID. That was finally successful and we were then able to delete my account. I appreciate your input which was helpful but wanted you to have these details in the event someone else might run into the same issue. Thanks!

Thanks for the reply! I do know my admin password but didn't try that on his account. I will give that a try.

Will try this tomorrow (iMac is at his studio). If it works, I will update here accordingly. I appreciate your feedback and am hoping this solves the issue.